Patent Publication Number: US-2023134660-A1

Title: Load monitoring, braking control, and height management

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of priority to U.S. Provisional Application Ser. No. 62/990,649, filed on Mar. 17, 2020, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Field of Invention 
     This disclosure relates to improvements in load monitoring and/or braking control systems for vehicles and/or trailers having one or more axles supported by air springs. 
     Discussion of the Background 
     Air suspension systems for vehicles have a plurality of air suspension springs or bags supporting one or more vehicle axles in pairs on either side of each axle. In one well-known vehicle, the pairs of air springs are connected by common large diameter air lines extending between correspondingly positioned air springs on adjacent axles. The common air lines are each connected by an air line to a height control valve directed to a respective side of a vehicle. The height control valve controls the air supply to the common air lines to adjust the inflation of the air springs to ensure that the vehicle is kept level as it is driven over variable road conditions. Unless defined otherwise, the term “height control valve” is used as equivalent to the term “leveling valve,” such that the terms “height control valve” and “leveling valve” may be used inter-changeably. 
     For example, when a vehicle negotiates a turn, the vehicle&#39;s center of gravity shifts along its width away from the turn. Due to the weight shift, the air springs on the side of the vehicle facing away from the turn start to contract, while the air springs on the side of the vehicle facing the turn start to extend. Consequently, the vehicle becomes unleveled from side-to-side. In response, one of the leveling valves on the lowered side of the vehicle supplies air to the contracted air springs, while the other leveling valve on the elevated side of the vehicle removes air from the extended air springs to keep the vehicle level. Through testing, it has now been found that leveling valves often overcompensate in responding to dynamic weight shifts of the vehicle, in which the air springs that were supplied air from the leveling valve tend to have a greater air pressure than the air springs that were purged by the leveling valve. As a result, a pressure difference persists between the two sides of the air suspensions system even after the leveling valves attempt to level the vehicle. Due to this pressure differential between the air springs, the vehicle remains unlevel even after the leveling valves have adjusted the pressure of the air springs in response to the vehicle weight shift. 
     Conventional load monitoring systems use pressure readings from one side of a vehicle or trailer to calculate weights (e.g., one or more axle masses and/or one or more axle group masses). When there is a pressure difference between the two sides, the pressures used to calculate weights will either be too high or too low and will result in inaccurate weight calculations. For instance, use of the lower pressure to calculate a weight will result in a weight calculation that is lower than the actual weight. This could results in fines for the vehicle operator (e.g., if the calculated weight is below a legal weight limit, but the actual weight is above the legal limit). Current technologies for measuring weights include on board mass units and load cells, which are costly to purchase and install and add complexity and maintenance costs to fleet operations. Further, despite their costs and complexities, such systems can produce inaccurate readings. 
     Conventional braking control systems may use pressure readings to calculate brake application levels. When there is a pressure difference between the two sides, the braking control system may use the higher pressure to calculate brake application levels. This results in the system applying the brakes harder than necessary, which results in unnecessary air usage, unnecessary excessive wear on the brakes and tires, and driver discomfort. 
     Accordingly, the present inventors have recognized that there is a need for load monitoring and/or braking control systems that address the various problems of existing systems. Moreover, the present inventors have recognized that with the system described herein, it is possible to eliminate various components and vehicle systems, thereby realizing significant cost savings in terms of vehicle procurement, maintenance, and lifetime operating performance metrics. 
     SUMMARY 
     The present invention addresses the problem of inaccurate pressure readings by using cross-flow pressure readings indicative of an air pressure within a cross-flow passage that connects first and second leveling valves that adjust independently a height of first and second sides, respectively, of a vehicle or a trailer. In some aspects, when the first and second leveling valves are adjusting independently the height of first and/or second sides, the first and second leveling valves may shut off the sides of the cross-flow passage, such that pressure is retained in the cross-flow passage from the preceding balanced state, i.e., the “balanced pressure” from a preceding “crossflow event.” This balanced pressure in the cross-flow passage has been found to be more accurate for the purposes of weight calculations and braking control (than using solely pressure readings from one or both sides of the vehicle or the trailer) while the vehicle or the trailer is changing dynamically (e.g., during axle input, body roll, turning, and/or braking). 
     One aspect of the present invention relates to a load monitoring system including a cross-flow passage, a cross-flow air pressure sensor, an analog-to-digital converter (ADC), and first and second pneumatic circuits. The cross-flow air pressure sensor may be configured to output cross-flow pressure information indicative of an air pressure within the cross-flow passage. The ADC may be configured to convert the cross-flow pressure information into digital cross-flow pressure information. The first pneumatic circuit may have a first leveling valve configured to adjust independently a height of a first side of a vehicle or a trailer. The second pneumatic circuit may have a second leveling valve configured to adjust independently a height of a second side of the vehicle or the trailer. The cross-flow passage may connect the first leveling valve and the second leveling valve. The first and second leveling valves may be configured to establish pneumatic communication between the first and second pneumatic circuits via the cross-flow passage when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side. The cross-flow pressure information is used for monitoring a load in the vehicle or the trailer. 
     In some aspects, the system may further include a display configured to display the digital cross-flow pressure information. In some aspects, the cross-flow air pressure sensor may be inside the cross-flow passage. In some aspects, the system may further include a fitting connected to the cross-flow passage, and the cross-flow air pressure sensor may be configured to communicate pneumatically with the cross-flow passage via the fitting. In some aspects, the fitting may be a T-fitting. In some aspects, the system may further include one or more air lines that connect the cross-flow air pressure sensor to the fitting for pneumatic communication between the cross-flow air pressure sensor and the cross-flow passage. 
     In some aspects, the system may further include (i) an air line connecting one or more air springs of the first pneumatic circuit and one or more air springs of the second pneumatic circuit and (ii) a fitting connected to the air line, and the cross-flow air pressure sensor may be configured to communicate pneumatically with the air line via the fitting. In some aspects, the air line connecting the one or more air springs of the first pneumatic circuit and the one or more air springs of the second pneumatic circuit may have a smaller diameter than the cross-flow passage. In some aspects, the fitting may be a T-fitting. In some aspects, the system may further include one or more air lines that connect the cross-flow air pressure sensor to the fitting for pneumatic communication between the cross-flow air pressure sensor and the air line connecting the one or more air springs of the first pneumatic circuit and the one or more air springs of the second pneumatic circuit. In some aspects, the air line may include a first back flow preventer on one side of the fitting and a second back flow preventer on the other side of the fitting, the first back flow preventer may be configured to prevent air from the one or more air springs of the second pneumatic circuit from flowing into the one or more air springs of the first pneumatic circuit via the air line, and the second back flow preventer may be configured to prevent air from the one or more air springs of the first pneumatic circuit from flowing into the one or more air springs of the second pneumatic circuit via the air line. In some aspects, the cross-flow pressure information may be indicative of the air pressure within the cross-flow passage when the first and second leveling valves have established pneumatic communication between the first and second pneumatic circuits via the cross-flow passage, and the cross-flow pressure information may be indicative of the higher of (i) an air pressure within one or more air springs of the first pneumatic circuit and (ii) an air pressure within one or more air springs of the second pneumatic circuit when the first and second leveling valves are adjusting independently the height of the first and second sides of the vehicle or the trailer. 
     In some aspects, the first pneumatic circuit may include a first air spring disposed on the first side, and the system may further include a first air spring air pressure sensor configured to output first air spring pressure information indicative of an air pressure within the first air spring. In some aspects, the system may further include an ADC and a display. The ADC may be configured to convert the first air spring pressure information into digital first air spring pressure information. The display may be configured to display the digital first air spring pressure information and the digital cross-flow pressure information. 
     In some aspects, the second pneumatic circuit comprises a second air spring disposed on the second side, and the system further comprises a second air spring air pressure sensor configured to output second air spring pressure information indicative of an air pressure within the second air spring. In some aspects, the system may further include an ADC and a display. The ADC may be configured to convert the second air spring pressure information into digital second air spring pressure information. The display may be configured to display the digital first air spring pressure information, the digital second air spring pressure information, and the digital cross-flow pressure information. 
     In some aspects, the system may further include a processor or computer configured to use the digital cross-flow pressure information to calculate a cross-flow-based weight on one or more axles of the vehicle or the trailer. In some aspects, the cross-flow pressure information may be first cross-flow pressure information indicative of an air pressure within the cross-flow passage at a first measurement time, and the cross-flow air pressure sensor may be further configured to output second cross-flow pressure information indicative of an air pressure within the cross-flow passage at a second measurement time. In some aspects, the digital cross-flow pressure information may be digital first cross-flow pressure information, and the ADC may be further configured to convert the second cross-flow pressure information into digital second cross-flow pressure information. In some aspects, the cross-flow-based weight may be indicative of a weight on the one or more axles of the vehicle or the trailer at the first measurement time, and the computer may be configured to use reference weight information, the digital first cross-flow pressure information, and the digital second cross-flow pressure information to calculate the cross-flow-based weight. In some aspects, the reference weight information is indicative of a weight on the one or more axles of the vehicle or the trailer at the second measurement time. In some aspects, the system may further include a display configured to display the cross-flow-based weight. 
     In some aspects, the first pneumatic circuit may include a first air spring disposed on the first side, and the system may further include a first air spring air pressure sensor configured to output first air spring pressure information indicative of an air pressure within the first air spring at the first measurement time and second air spring pressure information indicative of an air pressure within the first air spring at the second measurement time. In some aspects, the system may include an ADC configured to convert the first air spring pressure information into digital first air spring pressure information and to convert the second air spring pressure information into digital second air spring pressure information, and the computer may be further configured to use the reference weight information, the digital first air spring pressure information, and the digital second air spring pressure information to calculate a first air spring-based weight on the one or more axles of the vehicle or the trailer at the first measurement time. In some aspects, the second pneumatic circuit may include a second air spring disposed on the second side, and the system may further include a second air spring air pressure sensor configured to output third air spring pressure information indicative of an air pressure within the second air spring at the first measurement time and fourth air spring pressure information indicative of an air pressure within the second air spring at the first measurement time. 
     In some aspects, the system may include an ADC configured to convert the third air spring pressure information into third digital air spring pressure information and to convert the fourth air spring pressure information into fourth digital air spring pressure information, and the computer may be further configured to use the reference weight information, the third digital air spring pressure information, and the fourth digital air spring pressure information to calculate a second air spring-based weight on the one or more axles of the vehicle or the trailer at the first measurement time. In some aspects, the ADC configured to convert the first and second cross-flow pressure information, the ADC configured to convert the first and second air spring pressure information, and the ADC configured to convert the third and fourth air spring pressure information may be the same ADC. In some aspects, the ADC configured to convert the first and second cross-flow pressure information, the ADC configured to convert the first and second air spring pressure information, and the ADC configured to convert the third and fourth air spring pressure information may be different ADCs. In some aspects, the system may further include a display configured to display the cross-flow-based weight, the first air spring-based weight, and the second air spring-based weight. 
     In some aspects, the system may further include a display configured to display the cross-flow-based weight and the first air spring-based weight. In some aspects, using the reference weight information, the digital first cross-flow pressure information, and the digital second cross-flow pressure information to calculate the cross-flow-based weight on the one or more axles of the vehicle or the trailer at the first measurement time may include: using the reference weight information and the digital first cross-flow pressure information to calculate a pressure-to-weight conversion function, and using the pressure-to-weight conversion function and the digital second cross-flow pressure information to calculate the cross-flow-based weight on the one or more axles of the vehicle or the trailer at the first measurement time. 
     In some aspects, the reference weight information may be first reference weight information; the cross-flow air pressure sensor may be further configured to output third cross-flow pressure information indicative of an air pressure within the cross-flow passage at a third measurement time; the ADC configured to convert the first and second cross-flow pressure information may be further configured to convert the third cross-flow pressure information into third digital cross-flow pressure information; and the computer may be configured to use the first reference weight information, second reference weight information, the digital first cross-flow pressure information, the digital second cross-flow pressure information, and the third digital cross-flow pressure information to calculate the cross-flow-based weight on the one or more axles of the vehicle or the trailer at the first measurement time. The second reference weight information may be indicative of a weight on the one or more axles of the vehicle or the trailer at the third measurement time. In some aspects, using the first reference weight information, the second reference weight information, the digital first cross-flow pressure information, the digital second cross-flow pressure information, and the third digital cross-flow pressure information to calculate the cross-flow-based weight on the one or more axles of the vehicle at the first measurement time may include: using the first and second reference weight information and the first and third digital cross-flow pressure information to calculate a pressure-to-weight conversion function; and using the pressure-to-weight conversion function and the digital second cross-flow pressure information to calculate the cross-flow-based weight on the one or more axles of the vehicle or the trailer at the first measurement time. 
     Another aspect of the invention relates to a load monitoring method including using a cross-flow air pressure sensor to output cross-flow pressure information indicative of an air pressure within a cross-flow passage. The cross-flow passage may connect a first leveling valve of a first pneumatic circuit with a second leveling valve of a second pneumatic circuit, the first level circuit may be configured to adjust independently a height of a first side of a vehicle or a trailer, the second leveling valve may be configured to adjust independently a height of a second side of the vehicle or the trailer, and the first and second leveling valves may be configured to establish pneumatic communication between the first and second pneumatic circuits via the cross-flow passage when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side. The load monitoring method may include using an analog-to-digital converter (ADC) to convert the cross-flow pressure information into digital cross-flow pressure information. 
     In some aspects, the method may further include: using the first leveling valve to adjust independently the height of the first side of the vehicle or the trailer; and using the second leveling valve to adjust independently the height of the second side of the vehicle or the trailer. 
     In some aspects, the method may further include using the first and second leveling valves to establish pneumatic communication between the first and second pneumatic circuits via the cross-flow passage when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side. In some aspects, the method may further include using a display to display the digital cross-flow pressure information. In some aspects, the cross-flow air pressure sensor may be inside the cross-flow passage. 
     In some aspects, the first pneumatic circuit may include a first air spring disposed on the first side of the vehicle or the trailer, and the method further include using a first air spring air pressure sensor configured to output first air spring pressure information indicative of an air pressure within the first air spring. In some aspects, the method may further include: using an ADC to convert the first air spring pressure information into digital first air spring pressure information; and using a display to display the digital first air spring pressure information and the digital cross-flow pressure information. In some aspects, the second pneumatic circuit may include a second air spring disposed on the second side of the vehicle or the trailer, and the method may further include using a second air spring air pressure sensor to output second air spring pressure information indicative of an air pressure within the second air spring. In some aspects, the method may further include: using an ADC to convert the second air spring pressure information into digital second air spring pressure information, and using a display to display the digital first air spring pressure information, the digital second air spring pressure information, and the digital cross-flow pressure information. 
     In some aspects, the method may further include using a computer to use the digital cross-flow pressure information to calculate a cross-flow-based weight on one or more axles of the vehicle or the trailer. In some aspects, the cross-flow pressure information may be first cross-flow pressure information indicative of an air pressure within the cross-flow passage at a first measurement time, the digital cross-flow pressure information may be digital first cross-flow pressure information, the cross-flow-based weight may be indicative of a weight on the one or more axles of the vehicle or the trailer at the first measurement time, and the method may further include: using the cross-flow air pressure sensor to output second cross-flow pressure information indicative of an air pressure within the cross-flow passage at a second measurement time; and using the ADC to convert the second cross-flow pressure information into digital second cross-flow pressure information. The computer may use reference weight information, the digital first cross-flow pressure information, and the digital second cross-flow pressure information to calculate the cross-flow-based weight, and the reference weight information may be indicative of a weight on the one or more axles of the vehicle or the trailer at the second measurement time. 
     In some aspects, the method may further include using a display to display the cross-flow-based weight. In some aspects, the first pneumatic circuit may include a first air spring disposed on the first side of the vehicle or the trailer, and the method may further include: using a first air spring air pressure sensor to output first air spring pressure information indicative of an air pressure within the first air spring at the first measurement time; and using the first air spring air pressure sensor to output second air spring pressure information indicative of an air pressure within the first air spring at the second measurement time. In some aspects, the method may further include: using an ADC to convert the first air spring pressure information into digital first air spring pressure information and to convert the second air spring pressure information into digital second air spring pressure information; and using the computer to use the reference weight information, the digital first air spring pressure information, and the digital second air spring pressure information to calculate a first air spring-based weight on the one or more axles of the vehicle or the trailer at the first measurement time. In some aspects, the second pneumatic circuit may include a second air spring disposed on a second side of the vehicle or the trailer, and the method may further include: using a second air spring air pressure sensor to output third air spring pressure information indicative of an air pressure within the second air spring at the first measurement time; and using the second air spring air pressure sensor to output fourth air spring pressure information indicative of an air pressure within the second air spring at the first measurement time. 
     In some aspects, the method may further include: using an ADC to convert the third air spring pressure information into third digital air spring pressure information and to convert the fourth air spring pressure information into fourth digital air spring pressure information; and using the computer to use the reference weight information, the third digital air spring pressure information, and the fourth digital air spring pressure information to calculate a second air spring-based weight on the one or more axles of the vehicle or the trailer at the first measurement time. In some aspects, the method may further include displaying the cross-flow-based weight, the first air spring-based weight, and the second air spring-based weight. In some aspects, the method may further include displaying the cross-flow-based weight and the first air spring-based weight. 
     In some aspects, using the reference weight information, the digital first cross-flow pressure information, and the digital second cross-flow pressure information to calculate the cross-flow-based weight on the one or more axles of the vehicle or the trailer at the first measurement time may include: using the reference weight information and the digital first cross-flow pressure information to calculate a pressure-to-weight conversion function; and using the pressure-to-weight conversion function and the digital second cross-flow pressure information to calculate the cross-flow-based weight on the one or more axles of the vehicle or the trailer at the first measurement time. 
     In some aspects, the reference weight information may be first reference weight information, and the method may further include: using the cross-flow air pressure sensor to output third cross-flow pressure information indicative of an air pressure within the cross-flow passage at a third measurement time; using an ADC to convert the third cross-flow pressure information into third digital cross-flow pressure information; and using the computer to use the first reference weight information, the second reference weight information, the digital first cross-flow pressure information, the digital second cross-flow pressure information, and the third digital cross-flow pressure information to calculate the cross-flow-based weight on the one or more axles of the vehicle or the trailer at the first measurement time. The second reference weight information may be indicative of a weight on the one or more axles of the vehicle or the trailer at the third measurement time. In some aspects, using the first reference weight information, the second reference weight information, the digital first cross-flow pressure information, the digital second cross-flow pressure information, and the third digital cross-flow pressure information to calculate the cross-flow-based weight on the one or more axles of the vehicle or the trailer at the first measurement time may include: using the first and second reference weight information and the first and third digital cross-flow pressure information to calculate a pressure-to-weight conversion function; and using the pressure-to-weight conversion function and the digital second cross-flow pressure information to calculate the cross-flow-based weight on the one or more axles of the vehicle or the trailer at the first measurement time. 
     Yet another aspect of the invention relates to a braking control system including a cross-flow passage, one or more speed and/or acceleration sensors, a cross-flow air pressure sensor, a computer, and first and second pneumatic circuits. The one or more speed and/or acceleration sensors may be configured to output speed and/or acceleration information indicative of a speed and/or acceleration of a vehicle and/or a trailer. The cross-flow air pressure sensor may be configured to output cross-flow pressure information indicative of an air pressure within the cross-flow passage. The computer may be configured to use the speed and/or acceleration information and the cross-flow pressure information to calculate first and second brake application levels. The computer may be configured to apply the calculated first brake application level to a first brake on a first side of the vehicle or the trailer. The computer may be configured to apply the calculated second brake application level to a second brake on a second side of the vehicle or the trailer. The first pneumatic circuit may have a first leveling valve configured to adjust independently a height of the first side. The second pneumatic circuit may have a second leveling valve configured to adjust independently a height of the second side. The cross-flow passage may connect the first leveling valve and the second leveling valve, and the first and second leveling valves may be configured to establish pneumatic communication between the first and second pneumatic circuits via the cross-flow passage when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side. 
     In some aspects, the cross-flow air pressure sensor is inside the cross-flow passage. In some aspects, the system may further include a fitting connected to the cross-flow passage, and the cross-flow air pressure sensor may be configured to communicate pneumatically with the cross-flow passage via the fitting. In some aspects, the fitting may be a T-fitting. In some aspects, the system may further include one or more air lines that connect the cross-flow air pressure sensor to the fitting for pneumatic communication between the cross-flow air pressure sensor and the cross-flow passage. 
     In some aspects, the system may further include (i) an air line connecting one or more air springs of the first pneumatic circuit and one or more air springs of the second pneumatic circuit and (ii) a fitting connected to the air line, and the cross-flow air pressure sensor may be configured to communicate pneumatically with the air line via the fitting. In some aspects, the air line connecting the one or more air springs of the first pneumatic circuit and the one or more air springs of the second pneumatic circuit may have a smaller diameter than the cross-flow passage. In some aspects, the fitting may be a T-fitting. In some aspects, the system may further include one or more air lines that connect the cross-flow air pressure sensor to the fitting for pneumatic communication between the cross-flow air pressure sensor and the air line connecting the one or more air springs of the first pneumatic circuit and the one or more air springs of the second pneumatic circuit. In some aspects, the air line may include a first back flow preventer on one side of the fitting and a second back flow preventer on the other side of the fitting, the first back flow preventer may be configured to prevent air from the one or more air springs of the second pneumatic circuit from flowing into the one or more air springs of the first pneumatic circuit via the air line, and the second back flow preventer may be configured to prevent air from the one or more air springs of the first pneumatic circuit from flowing into the one or more air springs of the second pneumatic circuit via the air line. In some aspects, the cross-flow pressure information may be indicative of the air pressure within the cross-flow passage when the first and second leveling valves have established pneumatic communication between the first and second pneumatic circuits via the cross-flow passage, and the cross-flow pressure information may be indicative of the higher of (i) an air pressure within one or more air springs of the first pneumatic circuit and (ii) an air pressure within one or more air springs of the second pneumatic circuit when the first and second leveling valves are adjusting independently the height of the first and second sides of the vehicle or the trailer. 
     In some aspects, the first pneumatic circuit may include a first air spring disposed on the first side of the vehicle or the trailer, and the system may further include a first air spring air pressure sensor configured to output first air spring pressure information indicative of an air pressure within the first air spring. In some aspects, the computer may be configured to use the speed and/or acceleration information, the cross-flow pressure information, and the first air spring pressure information to calculate the first and second brake application levels. In some aspects, the second pneumatic circuit may include a second air spring disposed on the second side of the vehicle or the trailer, and the system may further include a second air spring air pressure sensor configured to output second air spring pressure information indicative of an air pressure within the second air spring. In some aspects, the computer may be configured to use the speed and/or acceleration information, the cross-flow pressure information, the first air spring pressure information, and the second air spring pressure information to calculate the first and second brake application levels. 
     In some aspects, the first and second brake application levels may be brake application pressure levels. In some aspects, the speed and/or acceleration information may include speed information indicative of the speed of the vehicle and/or the trailer and acceleration information indicative of an acceleration of the vehicle and/or the trailer, and the computer may be configured to use the speed information, the acceleration information, and the cross-flow pressure information to calculate the first and second brake application levels. In some aspects, the speed information may include wheel rotational speed information for one or more wheels of the vehicle or the trailer. In some aspects, the acceleration information may include lateral acceleration information. 
     In some aspects, the cross-flow passage may be a vehicle cross-flow passage, the cross-flow air pressure sensor may be a vehicle cross-flow air pressure sensor, the cross-flow pressure information may be vehicle cross-flow pressure information, the first and second brake application levels may be first and second vehicle brake application levels, and the first and second brakes may be first and second vehicle brakes. The computer may be further configured to: receive trailer cross-flow pressure information indicative of an air pressure within a trailer cross-flow passage of a trailer; use the speed and/or acceleration information, the vehicle cross-flow pressure information, and the trailer cross-flow pressure information to calculate the first and second vehicle brake application levels and first and second trailer brake application levels; apply the calculated first trailer brake application level to a first trailer brake on a first side of the trailer; and apply the calculated second trailer brake application level to a second trailer brake on a second side of the trailer. 
     In some aspects, the first and second pneumatic circuits may be first and second vehicle pneumatic circuits, and the system may further include: the trailer cross-flow passage; a trailer cross-flow air pressure sensor configured to output the trailer cross-flow pressure information indicative of the air pressure within the trailer cross-flow passage; a first trailer pneumatic circuit having a first trailer leveling valve configured to adjust independently a height of the first side of the trailer; and a second trailer pneumatic circuit having a second trailer leveling valve configured to adjust independently a height of the second side of the trailer. The trailer cross-flow passage may connect the first trailer leveling valve and the second trailer leveling valve, and the first and second trailer leveling valves may be configured to establish pneumatic communication between the first and second trailer pneumatic circuits via the trailer cross-flow passage when neither the first trailer leveling valve is adjusting independently the height of the first side of the trailer nor the second trailer leveling valve is adjusting independently the height of the second side of the trailer. In some aspects, the computer may be further configured to (i) receive a brake pedal pressure signal indicative of a pressure applied to a brake pedal of the vehicle and (ii) use the brake pedal pressure signal, the speed and/or acceleration information, the vehicle cross-flow pressure information, and the trailer cross-flow pressure information to calculate the first and second vehicle brake application levels and the first and second trailer brake application levels. 
     In some aspects, the computer may be further configured to (i) receive a brake pedal pressure signal indicative of a pressure applied to a brake pedal of the vehicle and (ii) use the brake pedal pressure signal, the speed and/or acceleration information, and the cross-flow pressure information to calculate the first and second brake application levels. 
     Still another aspect of the invention relates to a braking control method including using one or more speed and/or acceleration sensors to output speed and/or acceleration information indicative of a speed and/or acceleration of a vehicle and/or a trailer. The braking control method may include using a cross-flow air pressure sensor to output cross-flow pressure information indicative of an air pressure within a cross-flow passage connecting a first leveling valve of a first pneumatic circuit with a second leveling valve of a second pneumatic circuit. The first level circuit may be configured to adjust independently a height of a first side of the vehicle or the trailer, the second leveling valve may be configured to adjust independently a height of a second side of the vehicle or the trailer, and the first and second leveling valves may be configured to establish pneumatic communication between the first and second pneumatic circuits via the cross-flow passage when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side. The braking control method may include using a computer to use the speed and/or acceleration information and the cross-flow pressure information to calculate first and second brake application levels. The braking control method may include using the computer to apply the calculated first brake application level to a first brake on the first side of the vehicle or the trailer. The braking control method may include using the computer to apply the calculated second brake application level to a second brake on the second side of the vehicle or the trailer. 
     In some aspects, the method may further include: using the first leveling valve to adjust independently the height of the first side of the vehicle or the trailer; and using the second leveling valve to adjust independently the height of the second side of the vehicle or the trailer. In some aspects, the method may further include using the first and second leveling valves to establish pneumatic communication between the first and second pneumatic circuits via the cross-flow passage when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side. 
     In some aspects, the first pneumatic circuit may include a first air spring disposed on the first side of the vehicle or the trailer, and the method may further include using a first air spring air pressure sensor to output first air spring pressure information indicative of an air pressure within the first air spring. In some aspects, the method may further include using the computer to use the speed and/or acceleration information, the cross-flow pressure information, and the first air spring pressure information to calculate the first and second brake application levels. In some aspects, the second pneumatic circuit may include a second air spring disposed on the second side of the vehicle or the trailer, and the method may further include using a second air spring air pressure sensor to output second air spring pressure information indicative of an air pressure within the second air spring. In some aspects, the method may further include using the computer to use the speed and/or acceleration information, the cross-flow pressure information, the first air spring pressure information, and the second air spring pressure information to calculate the first and second brake application levels. 
     In some aspects, the first and second brake application levels may be brake application pressure levels. In some aspects, the speed and/or acceleration information may include speed information indicative of the speed of the vehicle and/or the trailer and acceleration information indicative of an acceleration of the vehicle and/or the trailer, and the computer may use the speed information, the acceleration information, and the cross-flow pressure information to calculate the first and second brake application levels. In some aspects, the speed information may include wheel rotational speed information for one or more wheels of the vehicle and/or the trailer. In some aspects, the acceleration information may include lateral acceleration information. 
     In some aspects, the cross-flow passage may be a vehicle cross-flow passage, the cross-flow air pressure sensor may be a vehicle cross-flow air pressure sensor, the cross-flow pressure information may be vehicle cross-flow pressure information, the first and second brake application levels may be first and second vehicle brake application levels, the first and second brakes may be first and second vehicle brakes, and the first and second pneumatic circuits may be first and second vehicle pneumatic circuits. The method may further include receiving trailer cross-flow pressure information indicative of an air pressure within a trailer cross-flow passage of a trailer. The trailer cross-flow passage may connect a first trailer leveling valve of a first trailer pneumatic circuit with a second trailer leveling valve of a second trailer pneumatic circuit, the first trailer level circuit may be configured to adjust independently a height of a first side of the trailer, the second trailer leveling valve may be configured to adjust independently a height of a second side of the trailer, and the first and second trailer leveling valves may be configured to establish pneumatic communication between the first and second trailer pneumatic circuits via the trailer cross-flow passage when neither the first trailer leveling valve is adjusting independently the height of the first side of the trailer nor the second trailer leveling valve is adjusting independently the height of the second side of the trailer. The method may further include using the speed and/or acceleration information, the vehicle cross-flow pressure information, and the trailer cross-flow pressure information to calculate the first and second vehicle brake application levels and first and second trailer brake application levels. The method may further include applying the calculated first trailer brake application level to a first trailer brake on the first side of the trailer. The method may further include applying the calculated second trailer brake application level to a second trailer brake on the second side of the trailer. 
     In some aspects, the method may further include using a trailer cross-flow air pressure sensor to output the trailer cross-flow pressure information indicative of the air pressure within the trailer cross-flow passage. In some aspects, the method may further include: using the first trailer pneumatic circuit having the first trailer leveling valve to adjust independently the height of the first side of the trailer; and using the second trailer pneumatic circuit having the second trailer leveling valve to adjust independently the height of the second side of the trailer. In some aspects, the method may further include using the first and second trailer leveling valves to establish pneumatic communication between the first and second trailer pneumatic circuits via the trailer cross-flow passage when neither the first trailer leveling valve is adjusting independently the height of the first side of the trailer nor the second trailer leveling valve is adjusting independently the height of the second side of the trailer. 
     In some aspects, the braking control method may further include using a computer to receive a brake pedal pressure signal indicative of a pressure applied to a brake pedal of the vehicle, and the computer uses the brake pedal pressure signal, the speed and/or acceleration information, and the cross-flow pressure information to calculate the first and second brake application levels. 
     Yet another aspect of the invention relates to a load monitoring system including a first pneumatic circuit, a second pneumatic circuit, an air pressure sensor, an analog-to-digital converter (ADC), and a computer. The first pneumatic circuit may have a first leveling valve configured to adjust independently a height of a first side of a vehicle or a trailer. The first pneumatic circuit may include a first air spring disposed on the first side. The second pneumatic circuit may have a second leveling valve configured to adjust independently a height of a second side of the vehicle or the trailer. The second pneumatic circuit may include a second air spring disposed on the second side. The air pressure sensor may be configured to output pressure information indicative of an air pressure that is different than both an air pressure within the first air spring and an air pressure within the second air spring at least when the first leveling valve is adjusting independently the height of the first side and the second leveling valve is adjusting independently the height of the second side. The ADC may be configured to convert the pressure information into digital pressure information. The computer may be configured to use the digital pressure information to calculate a weight on one or more axles of the vehicle or the trailer. The calculated weight may have less variation than an air spring-based weight calculated using pressure information indicative of an air pressure within the first air spring or the second air spring. 
     In some aspects, the system may further include a cross-flow passage that connects the first leveling valve and the second leveling valve, the pressure information output by the air pressure sensor may be indicative of an air pressure within the cross-flow passage, the calculated weight may be a cross-flow-based weight, and the first and second leveling valves may be configured to establish pneumatic communication between the first and second pneumatic circuits via the cross-flow passage when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side. In some aspects, a variation of the calculated weight may be less than 30% (and preferably less than 25% and more preferably less than 20%) of a variation of the air spring-based weight. 
     Still another aspect of the invention relates to a load monitoring method including using a first leveling valve of a first pneumatic circuit to adjust independently the height of a first side of the vehicle or the trailer. The first pneumatic circuit may include a first air spring disposed on the first side. The method may include using a second leveling valve of a second pneumatic circuit to adjust independently the height of a second side of the vehicle or the trailer. The second pneumatic circuit may include a second air spring disposed on the second side. The method may include using an air pressure sensor to output pressure information indicative of an air pressure that is different than both an air pressure within the first air spring and an air pressure within the second air spring at least when the first leveling valve is adjusting independently the height of the first side and the second leveling valve is adjusting independently the height of the second side. The method may include using an analog-to-digital converter (ADC) to convert the pressure information into digital pressure information. The method may include using a computer to use the digital pressure information to calculate a weight on one or more axles of the vehicle or the trailer, wherein the calculated weight has less variation than an air spring-based weight calculated using pressure information indicative of an air pressure within the first air spring or the second air spring. 
     In some aspects, the method may further include using the first and second leveling valves to establish pneumatic communication between the first and second pneumatic circuits via a cross-flow passage that connects the first leveling valve and the second leveling valve when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side, the pressure information output by the air pressure sensor may be indicative of an air pressure within the cross-flow passage, and the calculated weight may be a cross-flow-based weight. In some aspects, a variation of the calculated weight may be less than 30% (and preferably less than 25% and more preferably less than 20%) of a variation of the air spring-based weight. 
     Yet another aspect of the invention relates to a load monitoring system including a first pneumatic circuit, a second pneumatic circuit, an air pressure sensor, an analog-to-digital converter (ADC), and a computer. The first pneumatic circuit may have a first leveling valve configured to adjust independently a height of a first side of a vehicle or a trailer. The first pneumatic circuit may include a first air spring disposed on the first side. The second pneumatic circuit may have a second leveling valve configured to adjust independently a height of a second side of the vehicle or the trailer. The second pneumatic circuit may include a second air spring disposed on the second side. The air pressure sensor may be configured to output pressure information indicative of an air pressure that is different than both an air pressure within the first air spring and an air pressure within the second air spring at least when the first leveling valve is adjusting independently the height of the first side and the second leveling valve is adjusting independently the height of the second side. The ADC may be configured to convert the pressure information into digital pressure information. The computer may be configured to use the digital pressure information to calculate a weight on one or more axles of the vehicle or the trailer. The calculated weight is less affected by one or more events that cause the first and/or second leveling valves to adjust independently the height of the first and/or second sides of the vehicle or the trailer than an air spring-based weight calculated using pressure information indicative of an air pressure within the first air spring or the second air spring. 
     In some aspects, the system may further include a cross-flow passage that connects the first leveling valve and the second leveling valve, the pressure information output by the air pressure sensor may be indicative of an air pressure within the cross-flow passage, the calculated weight may be a cross-flow-based weight, and the first and second leveling valves may be configured to establish pneumatic communication between the first and second pneumatic circuits via the cross-flow passage when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side. In some aspects, the one or more events include non-zero lateral acceleration of the vehicle or the trailer. 
     Yet another aspect of the invention relates to a load monitoring method including using a first leveling valve of a first pneumatic circuit to adjust independently the height of a first side of the vehicle or the trailer. The first pneumatic circuit may include a first air spring disposed on the first side. The method may include using a second leveling valve of a second pneumatic circuit to adjust independently the height of a second side of the vehicle or the trailer. The second pneumatic circuit may include a second air spring disposed on the second side. The method may include using an air pressure sensor to output pressure information indicative of an air pressure that is different than both an air pressure within the first air spring and an air pressure within the second air spring at least when the first leveling valve is adjusting independently the height of the first side and the second leveling valve is adjusting independently the height of the second side. The method may include using an analog-to-digital converter (ADC) to convert the pressure information into digital pressure information. The method may include using a computer to use the digital pressure information to calculate a weight on one or more axles of the vehicle or the trailer. The calculated weight may be less affected by one or more events that cause the first and/or second leveling valves to adjust independently the height of the first and/or second sides of the vehicle or the trailer than an air spring-based weight calculated using pressure information indicative of an air pressure within the first air spring or the second air spring. 
     In some aspects, the method may further include using the first and second leveling valves to establish pneumatic communication between the first and second pneumatic circuits via a cross-flow passage that connects the first leveling valve and the second leveling valve when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side, and the pressure information output by the air pressure sensor may be indicative of an air pressure within the cross-flow passage, and the calculated weight is a cross-flow-based weight. In some aspects, the one or more events may include non-zero lateral acceleration of the vehicle or the trailer. 
     Still another aspect of the invention relates to a braking control system including a first pneumatic circuit, a second pneumatic circuit, one or more first brakes, and one or more second brakes. The first pneumatic circuit may have a first leveling valve configured to adjust independently a height of a first side of a vehicle or a trailer. The second pneumatic circuit may have a second leveling valve configured to adjust independently a height of a second side of the vehicle or the trailer. The one or more first brakes may be on the first side of the vehicle or the trailer. The one or more second brakes may be on the second side of the vehicle or the trailer. The braking control system may be configured to apply a brake application level to the one or more first brakes and the one or more second brakes, and the braking control system may be configured to apply only the same brake application level to both the one or more first brakes and the one or more second brakes. 
     In some aspects, the brake application level may be based on a pressure applied to a brake pedal. In some aspects, the system may further include a cross-flow passage that connects the first leveling valve and the second leveling valve, and the first and second leveling valves may be configured to establish pneumatic communication between the first and second pneumatic circuits via the cross-flow passage when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side. In some aspects, the brake application level may be based on only the pressure applied to the brake pedal. In some aspects, the brake application level may be based on (1) speed and/or acceleration information indicative of a speed and/or acceleration of the vehicle and/or the trailer and/or (2) pressure information indicative of an air pressure within (a) an air spring of the first or second pneumatic circuits or (b) a cross-flow passage connecting the first and second leveling valves. 
     Yet another aspect of the invention relates to a braking control method including using a first leveling valve of a first pneumatic circuit to adjust independently the height of a first side of the vehicle or the trailer. The method may include using a second leveling valve of a second pneumatic circuit to adjust independently the height of a second side of the vehicle or the trailer. The method may include applying a brake application level to (i) one or more first brakes on the first side of the vehicle or the trailer and (ii) one or more second brakes on the second side of the vehicle or the trailer, and only the same brake application level may be applied to both the one or more first brakes and the one or more second brakes. 
     In some aspects, the brake application level may be based on a pressure applied to a brake pedal. In some aspects, the method may further include using the first and second leveling valves to establish pneumatic communication between the first and second pneumatic circuits via a cross-flow passage that connects the first leveling valve and the second leveling valve when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side. In some aspects, the brake application level may be based on only the pressure applied to the brake pedal. In some aspects, the brake application level may be based on (1) speed and/or acceleration information indicative of a speed and/or acceleration of the vehicle and/or the trailer and/or (2) pressure information indicative of an air pressure within (a) an air spring of the first or second pneumatic circuits or (b) a cross-flow passage connecting the first and second leveling valves. 
     Still another aspect of the invention relates to a system including (1) a first pneumatic circuit and (2) a second pneumatic circuit. The first pneumatic circuit may include: (1A) a first air spring, (1B) a second air spring, (1C) a third air spring, (1D) a first air line, (1E) a second air line, (1F) a first leveling valve. The first air spring may be configured to support a first axle of a vehicle or a trailer on a first side of the vehicle or the trailer. The second air spring may be configured to support a second axle of the vehicle or the trailer on the first side of the vehicle or the trailer. The third air spring may be configured to support a third axle of the vehicle or the trailer on the first side of the vehicle or the trailer. The second axle may be located between the first and third axles. The first air line may connect the first and second air springs of the first pneumatic circuit. The second air line may connect second and third air springs of the first pneumatic circuit. The first leveling valve may be configured to adjust independently a height of the first side of the vehicle or the trailer by increasing or decreasing an amount of air in the first, second, and third air springs of the first pneumatic circuit. The second pneumatic circuit may include: (2A) a first air spring, (2B) a second air spring, (2C) a third air spring, (2D) a first air line, (2E) a second air line, and (2F) a second leveling valve. The first air spring of the second pneumatic circuit may be configured to support the first axle of the vehicle or the trailer on a second side of the vehicle or the trailer. The second air spring of the second pneumatic circuit may be configured to support the second axle of the vehicle or the trailer on the second side of the vehicle or the trailer. The third air spring of the second pneumatic circuit may be configured to support the third axle of the vehicle or the trailer on the second side of the vehicle or the trailer. The first air line of the second pneumatic circuit may connect the first and second air springs of the second pneumatic circuit. The second air line of the second pneumatic circuit may connect the second and third air springs of the second pneumatic circuit. The second leveling valve of the second pneumatic circuit may be configured to adjust independently a height of the second side of the vehicle or the trailer by increasing or decreasing an amount of air in the first, second, and third air springs of the second pneumatic circuit. 
     In some aspects, the first and second air lines of the first pneumatic circuit may be configured to enable front-to-back and/or back-to-front air flow between the first, second, and third air springs of the first pneumatic circuit, and the first and second air lines of the second pneumatic circuit may be configured to enable front-to-back and/or back-to-front air flow between the first, second, and third air springs of the second pneumatic circuit. In some aspects, the second air spring of the first pneumatic circuit may be configured to act as a reservoir between the first and third air springs of the first pneumatic circuit, and the second air spring of the second pneumatic circuit may be configured to act as a reservoir between the first and third air springs of the second pneumatic circuit. In some aspects, air added to the first and third air springs of the first pneumatic circuit may pass through the second air spring of the first pneumatic circuit before reaching the first and third air springs of the first pneumatic circuit, and air added to the first and third air springs of the second pneumatic circuit may pass through the second air spring of the second pneumatic circuit before reaching the first and third air springs of the second pneumatic circuit. In some aspects, the system may further include a cross-flow passage that connects the first leveling valve and the second leveling valve, and the first and second leveling valves are configured to establish pneumatic communication between the first and second pneumatic circuits via the cross-flow passage when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side. 
     In the various aspects of the invention set forth above, the first pneumatic circuit may include: (1A) a first air spring, (1B) a second air spring, (1C) a third air spring, (1D) a first air line, (1E) a second air line, (1F) a first leveling valve. The first air spring may be configured to support a first axle of a vehicle or a trailer on a first side of the vehicle or the trailer. The second air spring may be configured to support a second axle of the vehicle or the trailer on the first side of the vehicle or the trailer. The third air spring may be configured to support a third axle of the vehicle or the trailer on the first side of the vehicle or the trailer. The second axle may be located between the first and third axles. The first air line may connect the first and second air springs of the first pneumatic circuit. The second air line may connect second and third air springs of the first pneumatic circuit. The first leveling valve may be configured to adjust independently a height of the first side of the vehicle or the trailer by increasing or decreasing an amount of air in the first, second, and third air springs of the first pneumatic circuit. The second pneumatic circuit may include: (2A) a first air spring, (2B) a second air spring, (2C) a third air spring, (2D) a first air line, (2E) a second air line, and (2F) a second leveling valve. The first air spring of the second pneumatic circuit may be configured to support the first axle of the vehicle or the trailer on a second side of the vehicle or the trailer. The second air spring of the second pneumatic circuit may be configured to support the second axle of the vehicle or the trailer on the second side of the vehicle or the trailer. The third air spring of the second pneumatic circuit may be configured to support the third axle of the vehicle or the trailer on the second side of the vehicle or the trailer. The first air line of the second pneumatic circuit may connect the first and second air springs of the second pneumatic circuit. The second air line of the second pneumatic circuit may connect the second and third air springs of the second pneumatic circuit. The second leveling valve of the second pneumatic circuit may be configured to adjust independently a height of the second side of the vehicle or the trailer by increasing or decreasing an amount of air in the first, second, and third air springs of the second pneumatic circuit. The first leveling valve may be configured to adjust independently the height of the first side of the vehicle or the trailer by increasing or decreasing an amount of air in the first, second, and third air springs of the first pneumatic circuit. The second leveling valve may be configured to adjust independently the height of the second side of the vehicle or the trailer by increasing or decreasing an amount of air in the first, second, and third air springs of the second pneumatic circuit. 
     In the various aspects of the invention set forth above, the system may further include one or more raise and lower valves configured to enable an operator to manually vary a height of a chassis of the vehicle trailer relative to the ground. In the various aspects of the invention set forth above, the system may further include one or more pressure protection valves. In the various aspects of the invention set forth above, the system may further include one or more dump valves. In the various aspects of the invention set forth above, the system may further include an integrated pressure protection valve unit including an inlet, a first outlet, a second outlet, and an exhaust. 
     Yet another aspect of the invention relates to a load monitoring system including a cross-flow passage, an air pressure sensor, a first pneumatic circuit, a second pneumatic circuit, an air line, a fitting, a first back flow preventer, and a second back flow preventer. The air pressure sensor may be configured to output pressure information indicative of an air pressure. The first pneumatic circuit may include one or more first air springs and a first leveling valve configured to adjust independently a height of a first side of a vehicle or a trailer. The second pneumatic circuit may include one or more second air springs and a second leveling valve configured to adjust independently a height of a second side of the vehicle or the trailer. The cross-flow passage may connect the first leveling valve and the second leveling valve. The first and second leveling valves may be configured to establish pneumatic communication between the first and second pneumatic circuits via the cross-flow passage when neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side. The air line may connect the one or more first air springs and the one or more second air springs. The fitting may be connected to the air line. The air pressure sensor may be configured to communicate pneumatically with the air line via the fitting. The first back flow preventer may be on one side of the fitting. The first back flow preventer may be configured to prevent air from the one or more air springs of the second pneumatic circuit from flowing into the one or more air springs of the first pneumatic circuit via the air line. The second back flow preventer may be on the other side of the fitting. The second back flow preventer may be configured to prevent air from the one or more air springs of the first pneumatic circuit from flowing into the one or more air springs of the second pneumatic circuit via the air line. 
     In some aspects, the air line connecting the one or more first air springs and the one or more second air springs may have a smaller diameter than the cross-flow passage. In some aspects, the fitting may be a T-fitting. In some aspects, the system may further include one or more air lines that connect the air pressure sensor to the fitting for pneumatic communication between the air pressure sensor and the air line connecting the one or more air springs of the first pneumatic circuit and the one or more air springs of the second pneumatic circuit. In some aspects, the pressure information may be indicative of the air pressure within the cross-flow passage when the first and second leveling valves have established pneumatic communication between the first and second pneumatic circuits via the cross-flow passage, and the pressure information is indicative of the higher of (i) an air pressure within the one or more first air springs and (ii) an air pressure within the one or more second air springs when the first and second leveling valves are adjusting independently the height of the first and second sides of the vehicle or the trailer. 
     Still another aspect of the invention relates to a system including a first leveling valve, a second leveling valve, and a raise lower valve (RLV). The first leveling valve may be configured to adjust independently a height of a first side of a vehicle or a trailer. The second leveling valve may be configured to adjust independently a height of a second side of the vehicle or the trailer. The RLV may be configured to, in response to a leveling valve height control input, allow the first and second leveling valves to adjust independently the heights of the first and second sides of the vehicle or the trailer. The RLV may be configured to, in response to a raise input, raise the height of one or both of the first and second sides of the vehicle or the trailer and prevent the first and second leveling valves from adjusting independently the heights of the first and second sides of the vehicle or the trailer. The RLV may be configured to, in response to a lower input, lower the height of one or both of the first and second sides of the vehicle or the trailer and prevent the first and second leveling valves from adjusting independently the heights of the first and second sides of the vehicle or the trailer. 
     Yet another aspect of the invention relates to a method performed by a raise lower valve (RLV). The method may include, in response to a leveling valve height control input, allowing a first leveling valve to adjust independently a height of a first side of a vehicle or a trailer and a second leveling valve to adjust independently a height of a second side of the vehicle or the trailer. The method may include, in response to a raise input, raising the height of one or both of the first and second sides of the vehicle or the trailer and preventing the first and second leveling valves from adjusting independently the heights of the first and second sides of the vehicle or the trailer. The method may include, in response to a lower input, lowering the height of one or both of the first and second sides of the vehicle or the trailer and preventing the first and second leveling valves from adjusting independently the heights of the first and second sides of the vehicle or the trailer. 
     Still another aspect of the invention relates to a raise lower valve (RLV) including one or more housings including one or more passages, one or more supply seals in the one or more passages, one or more delivery seals in the one or more passages, a first actuator, a second actuator, and a controller. The first actuator may be configured to move the one or more supply seals in the one or more passages. The second actuator may be configured to move the one or more delivery seals in the one or more passages. The controller may be configured to control the first and second actuators to move the one or more supply seals and the one or more delivery seals in the one or more passages. 
     Yet another aspect of the invention relates to a method performed by a raise lower valve (RLV). The method may include using a controller to control a first actuator to move one or more supply seals in one or more passages of one or more housings. The method may include using the controller to control a second actuator to move one or more delivery seals in the one or more passages. 
     Still another aspect may provide an air management system for a vehicle or trailer. The air management system may include a first pneumatic circuit having a first leveling valve configured to adjust independently a height of a first side of a first axle of the vehicle or the trailer. The air management system may include a second pneumatic circuit having a second leveling valve configured to adjust independently a height of a second side of the first axle of the vehicle or the trailer. The air management system may include a first cross-flow line connecting the first leveling valve with the second leveling valve. The air management system may include a third pneumatic circuit having a third leveling valve configured to adjust independently a height of a third axle of the vehicle or the trailer. The air management system may include a second cross-flow line connecting the third leveling valve with the first cross-flow line. The first, second, and third leveling valves may be configured to establish pneumatic communication between the first, second, and third pneumatic circuits when the first leveling valve is not independently adjusting the height of the first side of the first axle, the second leveling valve is not independently adjusting the height of the second side of the first axle, and the third leveling valve is not independently adjusting the height of the second axle. 
     In some aspects, the first leveling valve may be further configured to adjust independently a height of a first side of a third axle of the vehicle or the trailer, and the second leveling valve may be further configured to adjust independently a height of a second side of the third axle. 
     Further variations encompassed within the systems and methods are described in the detailed description of the invention below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate various, non-limiting aspects of the present invention. In the drawings, like reference numbers indicate identical or functionally similar elements. 
         FIGS.  1 A and  1 B  are side views illustrating a vehicle and trailer embodying aspects of the present invention. 
         FIGS.  2 A,  2 B, and  2 C  are block diagrams illustrating vehicle systems embodying aspects of the present invention. 
         FIGS.  2 D,  2 E,  2 F,  2 G, and  2 O  are block diagrams illustrating trailer portions of vehicle systems embodying aspects of the present invention. 
         FIGS.  2 H,  2 I,  2 J,  2 K,  2 L,  2 M,  2 N,  2 P, and  2 Q  are block diagrams illustrating pneumatic circuits of trailer portions of vehicle systems embodying aspects of the present invention. 
         FIG.  2 R  is a block diagram illustrating a trailer portion of a vehicle system embodying aspects of the present invention. 
         FIGS.  2 S- 2 U  illustrate a side view, top view, and block diagram, respectively, of a two axle trailer embodying aspects of the present invention. 
         FIGS.  2 V- 2 X  illustrate a side view, top view, and block diagram, respectively, of a three axle trailer embodying aspects of the present invention. 
         FIGS.  3 A,  3 B,  3 C,  3 D, and  3 E  are block diagram illustrating cross-flow air pressure sensors embodying aspects of the present invention. 
         FIG.  4    is a block diagram illustrating a computer of a vehicle system embodying aspects of the present invention. 
         FIG.  5 A  is a flow chart illustrating a braking control process embodying aspects of the present invention. 
         FIG.  5 B  is a flow chart illustrating a braking control process embodying aspects of the present invention. 
         FIG.  6    is a flow chart illustrating a load monitoring process embodying aspects of the present invention. 
         FIG.  7    is a flow chart illustrating a load monitoring process embodying aspects of the present invention. 
         FIGS.  8 A- 8 E  are block diagrams illustrating a raise and lower valve embodying aspects of the present invention. 
         FIG.  9    is a flow chart illustrating a raise lower process embodying aspects of the present invention. 
         FIG.  10    is a block diagram illustrating a raise and lower valve embodying aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     While the present invention may be embodied in many different forms, a number of illustrative aspects are described herein with the understanding that the present disclosure is to be considered as providing examples of the principles of the invention and such examples are not intended to limit the invention to preferred aspects described herein and/or illustrated herein. 
     The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. 
     In understanding the scope of the present disclosure, the terms “including” or “comprising” and their derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms “including”, “having” and their derivatives. The term “consisting” and its derivatives, as used herein, are intended to be closed terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The term “consisting essentially of”, as used herein, is intended to specify the presence of the stated features, elements, components, groups, integers, and/or steps as well as those that do not materially affect the basic and novel characteristic(s) of features, elements, components, groups, integers, and/or steps. It is understood that reference to any one of these transition terms (i.e. “comprising,” “consisting,” or “consisting essentially”) provides direct support for replacement to any of the other transition term not specifically used. For example, amending a term from “comprising” to “consisting essentially of” would find direct support due to this definition. 
       FIG.  1 A  illustrates a non-limiting aspect of a vehicle  10  (e.g., a car, truck, tractor, or load carrying prime mover) embodying aspects of the present invention.  FIG.  1 B  illustrates an alternative aspect in which the vehicle  10  is a semi-tractor. In some aspects, the vehicle  10  may include one or more wheeled axles (e.g., one or more of axles  12 ,  14 , and  15 ). In some aspects, the axles may be driven or non-driven. In some aspects, the vehicle  10  may include one or more steering axles  12  (e.g., at the front of the vehicle  10 ) and one or more drive axles  14  and  15 . In some aspects, as shown in the  FIGS.  1 A and  1 B , the vehicle  10  may include one or more trailers  20 . In some aspects, a trailer  20  may be a towable of any kind. In some aspects, the trailer  20  may have one or more wheeled axles (e.g., one or more of axles  16 ,  18 ,  19 , and  21 ). In some aspects, the trailer axles may include one or more steering axles  16  (e.g., at the front of the trailer  20 ) and/or one or more drive axles  18 ,  19 , and  21 . In some aspects, the vehicle  10  and trailer  20  may communicate via a trailer interface  143 , which may include, for example and without limitation, a trailer plug and socket. 
       FIG.  2 A  is a block diagram of a non-limiting aspect of a vehicle system  100 . In some aspects, the system  100  may provide braking control for the vehicle  10  and/or trailer  20 . In some aspects, the system  100  may additionally or alternatively provide load monitoring for the vehicle  10  and/or trailer  20 . In some aspects, the system  100  may additionally or alternatively provide air management for the vehicle  10  and/or trailer  20 . 
     In some aspects, the system  100  may include a first pneumatic circuit  102  and a second pneumatic circuit  104 . The first pneumatic circuit  102  may include one or more first air springs  106  (e.g., one or more air bags) and a first leveling valve  108 . The second pneumatic circuit  104  may include one or more second air springs  110  and a second leveling valve  112 . In some aspects, the first leveling valve  108  may be configured to adjust independently the height of a first side of the vehicle  10  (e.g., by increasing or decreasing air in the one or more first air springs  106 ), and the second leveling valve  112  may be configured to adjust independently the height of a second side of the vehicle  10  (e.g., by increasing or decreasing air in the one or more second air springs  110 ). In some aspects, the first and second sides may be opposite sides of the vehicle  10  (e.g., left and right sides). In some aspects, the system  100  may include one or more air supply tanks  114 . In some aspects, one or more supply lines may connect the one or more air supply tanks  114  to the first leveling valve  108 , and one or more supply lines may connect the one or more air supply tanks  114  to the second leveling valve  112 . 
     In some aspects, the first leveling valve  108  may increase air in the one or more first air springs  106  by supplying air from one or more of the one or more air supply tanks  114 , and the second leveling valve  112  may increase air in the one or more second air springs  110  by supplying air from one or more of the one or more air supply tanks  114 . In some aspects, the first leveling valve  108  may decrease air in the one or more first air springs  106  by purging air from the one or more first air springs  106  (e.g., allowing air from the one or more first air springs  106  out of an exhaust port), and the second leveling valve  112  may decrease air in the one or more second air springs  110  by purging air from the one or more second air springs  110  (e.g., allowing air from the one or more second air springs  110  out of an exhaust port). 
     In some aspects, as shown in  FIG.  2 A , the system  100  may include one or more raise and lower valves (RLVs)  107 . In some aspects, the one or more RLVs  107  may enable an operator to manually vary the height of the chassis of the vehicle  10  relative to the ground (e.g., to facilitate height adjustment of the vehicle at the loading dock). In some aspects, although the RLV  107  is shown as two units in  FIG.  2 A , the RLV  107  may be a single unit so that the operator may manually vary the height of the chassis of the vehicle  10  relative to the ground on both sides of the vehicle  10  using a single controller. In some alternative aspects, the system  100  may include a first RLV  107  for manually varying the height of the chassis of the vehicle  10  on the first side of the vehicle  10 , and the system  100  may include a second RLV  107  for manually varying the height of the chassis of the vehicle  10  on the second side of the vehicle  10 . 
     In some aspects, the one or more RLVs  107  may be connected by air lines to the first leveling valve  108 , the one or more first air springs  106 , the second leveling valve  112 , and the one or more second air springs  110 . In some aspects, although not shown in  FIG.  2 A , one or more supply lines may supply air from the one or more air supply tanks  114  to the one or more RLVs  107  (e.g., for increasing the height of the chassis of the vehicle  10  relative to the ground). In some aspects, when set in a neutral or drive mode (e.g., while the vehicle  10  is traveling), the one or more RLVs  107  may enable pneumatic communication between the first leveling valve  108  and the one or more first air springs  106  and pneumatic communication between the second leveling valve  112  and the one or more second air springs  110  so that the first and second leveling valves  108  and  112  perform automatic height control by controlling the amounts of air in the first and second air springs  106  and  110 , respectively. 
     In some aspects, the operator may use the one or more RLVs  107  to manually control the height of the chassis of the vehicle  10  relative to the ground when first and second leveling valves  108  and are not performing automatic height control (e.g., while the vehicle  10  is stationary, such as at a loading dock). In some aspects, when the operator is manually controlling the height of the chassis of the vehicle  10  relative to the ground, the one or more RLVs  107  may be set to one of a stop mode, a raise mode, and a lower mode. In some aspects, in each of the stop, raise, and lower modes, the one or more RLVs  107  may prevent pneumatic communication between the first leveling valve  108  and the one or more first air springs  106  and prevent pneumatic communication between the second leveling valve  112  and the one or more second air springs  110 . In some aspects, when set in the stop mode, the one or more RLVs  107  may prevent all pneumatic communication through the one or more RLVs  107  so that height of the chassis of the vehicle  10  relative to the ground does not change. In some aspects, when set in the raise mode, the one or more RLVs  107  may enable pneumatic communication between the one or more air supply tanks  114  and the one or more first air springs  106  and pneumatic communication between the one or more air supply tanks  114  and the one or more second air springs  110  increase the amount of air in the first and second air springs  106  and  110  and, thereby, increase the height of the chassis of the vehicle  10 . In some aspects, when set in the lower mode, the one or more RLVs  107  may vent air from the one or more first air springs  106  and the one or more second air springs  110  to decrease the amount of air in the first and second air springs  106  and  110  and, thereby, decrease the height of the chassis of the vehicle  10 . In some aspects, in all of the RLV modes, the one or more RLVs  107  may prevent the one or more first air springs  106  from communicating pneumatically with the second leveling valve  112  and the one or more second air springs  110  and may prevent the first leveling valve  108  from communicating pneumatically with the second leveling valve  112  and the one or more second air springs  110 . 
     However, one or more RLVs  107  are not required, and, in some alternative aspects, the system  100  may not include an RLV, one or more air lines may connect directly the first leveling valve  108  and the one or more first air springs  106 , and one or more air lines may connect directly the second leveling valve  112  and the one or more second air springs  110 . In some aspects, the system  100  may include one or more dump valves. In some aspects, one or more air lines may connect and provide pneumatic communication (i) between a dump port of the first leveling valve  108  to the dump valve and (ii) between a dump port of the second leveling valve  112  to the dump valve. In some aspects, an air line may connect and provide pneumatic communication between the one or more air supply tanks  114  and the dump valve. In some aspects, the dump valve may be used to dump (manually or automatically) air from the air springs  106  and  110 . 
     In some aspects, the system  100  may include one or more pressure protection valves (PPVs), which may protect the system  100  in the event of a leak or failure within the system  100 . For example, as shown in  FIG.  2 A , the system  100  may include a first PPV  115  between the one or more air supply tanks  114  and the first leveling valve  108  and a second PPV  117  between the one or more air supply tanks  114  and the second leveling valve  112 . In some aspects, a supply line may connect the one or more air supply tanks  114  to the first PPV  115 , and a supply line may connect the first PPV  115  and the first leveling valve  108 . In some aspects, a supply line may connect the one or more air supply tanks  114  to the second PPV  117 , and a supply line may connect the second PPV  117  and the second leveling valve  112 . In some alternative aspects, the first and second PPVs  115  and  117  may be integrated into a single PPV unit (e.g., a dual PPV including an inlet, two outlets, and an exhaust), the one or more air supply tanks  114  may be connected to an inlet of the integrated PPV unit, a first air line may connect a first outlet of the integrated PPV unit and the first leveling valve  108 , and a second air line may connect a second outlet of the integrated PPV unit and the second leveling valve  112 . However, the one or more PPVs are not required, and, in some alternative aspects, a supply line may connect the one or more air supply tanks  114  directly to the first leveling valve  108 , and a supply line may connect the one or more air supply tanks  114  directly to the second leveling valve  112 . 
     In some aspects, the air lines of the first and second pneumatic circuits  102  and  104  may be configured to supply equal volumes of air to maintain symmetry within the pneumatic circuits  102  and  104  on both sides of the vehicle  10 . In some aspects, the air lines of the first and second pneumatic circuits  102  and  104  may include (i) an air line connecting the first leveling valve  108  and the RLV  107  and one or more air lines connecting the RLV  107  and the one or more first air springs  106  (or one or more air lines connecting the first leveling valve  108  and the one or more first air springs  106  if there is no RLV  107 ), (ii) an air line connecting the second leveling valve  112  and the RLV  107  and one or more air lines connecting the RLV  107  and the one or more second air springs  110  (or one or more air lines connecting the second leveling valve  112  and the one or more second air springs  110  if there is no RLV  107 ), (iii) a supply line connecting the one or more air supply tanks  114  to a PPV (e.g., the first PPV  115 ) and a supply line may connect the PPV and the first leveling valve  108  (or one or more supply lines connecting the one or more air supply tanks  114  and the first leveling valve  108  if there is no PPV), and/or (iv) a supply line connecting the one or more air supply tanks  114  to a PPV (e.g., the second PPV  117 ) and a supply line may connect the PPV and the second leveling valve  112  (or one or more supply lines connecting the one or more air supply tanks  114  and the second leveling valve  112  if there is no PPV). In some aspects, one or more of the air lines of the first pneumatic circuit  102  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as one or more of the corresponding air lines of the second pneumatic circuit  104 . 
     The present invention is not limited to having the particular number of axle(s)  12 ,  14 , and/or  15 , air springs  106  and  110 , air lines/hoses, and/or air supply tank(s)  114  that are shown in the drawings, as these elements vary depending on the type of vehicle that is used as would be immediately clear to a person skilled in the art. In some aspects, as shown in  FIG.  2 A , the first pneumatic circuit  102  and a second pneumatic circuit  104  may be supplied air by one or more common air supply tanks  114 . However, this is not required, and, in some alternative aspects, one or more air supply tanks may be dedicated to the first pneumatic circuit  102 , and one or more different air supply tanks may be dedicated to the second pneumatic circuit  104 . 
     In some aspects, the system  100  may include a cross-flow passage  116  that connects the first leveling valve  108  with the second leveling valve  112 . In some aspects, the cross-flow passage  116  may be an air line. The first and second leveling valves  108  and  112  may establish pneumatic communication between the first and second pneumatic circuits  102  and  104  when neither the first leveling valve  108  is adjusting independently the height of the first side of the vehicle  10  nor the second leveling valve  112  is adjusting independently the height of the second side of the vehicle  10  (e.g., when (i) the first leveling valve  108  is neither supplying air to nor purging air from the one or more first air springs  106  and (ii) the second leveling valve  112  is neither supplying air to nor purging air from the one or more second air springs  110 ). In some aspects, the cross-flow passage  116  is not directly connected to the one or more air supply tanks  114  (or to a supply line connected to an air supply tank  114 ). In some aspects, the pneumatic communication between the first and second pneumatic circuits  102  and  104  via the cross-flow passage  116  may equalize air pressure between the one or more first air springs  106  and the one or more second air springs  110 . As a result, the first and second leveling valves  108  and  112  link together the first and second pneumatic circuits  102  and  104  as a common circuit when neither the first leveling valve  108  is adjusting independently the height of the first side of the vehicle  10  nor the second leveling valve  112  is adjusting independently the height of the second side of the vehicle  10 . 
     Accordingly, during a shift in the center of gravity of the vehicle  10  (e.g., when the vehicle  10  is negotiating a sharp turn and/or is on uneven ground), one of the first and second leveling valves  108  and  112  may supply air to the one of the first and second air springs  106  and  110  that have been contracted from the weight shift of the vehicle  10  while the other one of the first and second leveling valves  108  and  112  purges air from the other one of the first and second air springs  106  and  110  that have been extended from the weight shift of the vehicle  10  without any cross-flow between the first and second pneumatic circuits  102  and  104 . In this state, the first and second leveling valves  108  and  112  may overcompensate for the dynamic weight shift of the vehicle  10  by either supplying too much air to one of the first and second air springs  106  and  110  or removing too much air from the other of the first and second air springs  106  and  110 , resulting in a slight pressure difference between the first and second air springs  106  and  110 . When the shift in the center of gravity of the vehicle  10  ends, if not for the first and second leveling valves  108  and  112  establishing pneumatic communication between the first and second pneumatic circuits  102  and  104 , the slight pressure difference would keep the vehicle  10  in an uneven state. However, because the first and second leveling valves  108  and  112  establish pneumatic communication between the first and second pneumatic circuits  102  and  104  when neither the first leveling valve  108  is adjusting independently the height of the first side of the vehicle  10  nor the second leveling valve  112  is adjusting independently the height of the second side of the vehicle  10 , the slight pressure difference between the first and second air springs  106  and  110  is eliminated as air passes via the cross-flow passage  116  from the one of the first and second air springs  106  and  110  at higher pressure to the other of the first and second air springs  106  and  110  at lower pressure, thereby reaching an equilibrium state. 
     In some aspects, the system  100  may include a cross-flow air pressure sensor  118 . In some aspects, the cross-flow air pressure sensor  118  may be configured to output cross-flow pressure information (e.g., an analog electrical signal) indicative of an air pressure within the cross-flow passage  116 . In some aspects, as shown in  FIG.  3 A , the cross-flow air pressure sensor  118  may be inside the cross-flow passage  116 . In some alternative aspects, as shown in  FIGS.  3 B and  3 C , the system  100  may include a fitting  340  connected to the cross-flow passage  116 , and the cross-flow air pressure sensor  118  may be configured to communicate pneumatically with the cross-flow passage  116  via the fitting  340 . In some aspects, the fitting  340  may be, for example and without limitation, a T-fitting. In some aspects, one or more air lines  342  may connect the cross-flow air pressure sensor  118  to the fitting  340  for pneumatic communication between the cross-flow air pressure sensor  118  and the cross-flow passage  116 . 
     In some alternative aspects, as shown in  FIGS.  3 D and  3 E , the system  100  may include an air line  348  connecting the one or more first air springs  106  of the first pneumatic circuit  102  and the one or more second air springs  110  of the second pneumatic circuit  104 . In some aspects, the air line  348  may have a smaller diameter than the cross-flow passage  116 . In some aspects, a fitting  350  may be connected to the air line  348 , and the cross-flow air pressure sensor  118  may be configured to communicate pneumatically with the air line  348  via the fitting  350 . In some aspects, the fitting  350  may be, for example and without limitation, a T-fitting. In some aspects, one or more air lines  352  may connect the cross-flow air pressure sensor  118  to the fitting  350  for pneumatic communication between the cross-flow air pressure sensor  118  and the air line  348 . In some aspects, the cross-flow air pressure sensor  118  that is in pneumatic communication with the air line  348 , which is connected to the first and second air springs  106  and  110 , may output cross-flow pressure information that is indicative of an air pressure in the air line  348 . In some aspects, cross-flow pressure information generated using the air line  348  may be more stable than cross-flow pressure information generated directly from the cross-flow passage  116 . 
     In some aspects, as shown in  FIGS.  3 D and  3 E , the air line  348  may include back flow preventers  349  and  351 . In some aspects, the back flow preventers  349  and  351  may be on opposite sides of the fitting  350 . In some aspects, the back flow preventer  349  may prevent air from the one or more second air springs  110  of the second pneumatic circuit  104  from flowing into the one or more first air springs  106  of the first pneumatic circuit  102  via the air line  348 . In some aspects, the back flow preventer  351  may prevent air from the one or more first air springs  106  of the first pneumatic circuit  102  from flowing into the one or more second air springs  110  of the second pneumatic circuit  104  via the air line  348 . In some aspects, the back flow preventers  349  and  351  may be double check valves, reduced pressure zone devices, or any other suitable devices for preventing back flow. 
     In some aspects, a residual pressure purge device (e.g., a purge solenoid) may be included to vent residual pressure from the air line  348 . In some aspects, the residual pressure purge device may prevent the back flow preventers  349  and  351  from creating a closed circuit from which pressure cannot escape. In some aspects, the residual pressure purge device may allow for system reset via a manual user input (e.g., a button next to the display  137 ), an input received (e.g., wirelessly) using the communication unit  139 , and/or automatically under control of the computer  124 . In some aspects including the residual pressure purge device, the fitting  350  may be a four-way fitting (e.g., instead of a three-way, T-fitting) connected to the backflow preventer  349 , backflow preventer  351 , cross-flow air pressure sensor  118 , and the residual pressure purge device. 
     In some aspects, in a balanced state where the air pressure in the first and second air springs  106  and  110  is the same (e.g., when none of the first and second leveling valves  108  and  112  is adjusting independently the height of the first and second sides of the vehicle  10  and the first and second leveling valves  108  and  112  have established pneumatic communication between the first and second pneumatic circuits  102  and  104  via the cross-flow passage  116  to eliminate even a slight pressure difference between the first and second air springs  106  and  110 ), the air pressure in the air line  348  may be the same as air pressure in the cross-flow passage  116 , which would be the same as the air pressure in the first and second air springs  106  and  110 . In balanced state, the cross-flow air pressure sensor  118  that is in pneumatic communication with the air line  348  may output cross-flow pressure information that is indicative of an air pressure in the cross-flow passage  116 . In some aspects, in an imbalanced state where the air pressure in the one or more air springs  106  is different than the air pressure in the one or more air springs  110  (e.g., when the first and second leveling valves  108  and  112  are adjusting independently the height of the first and second sides of the vehicle  10 ), the back flow preventers  349  and  351  may result in the air pressure in the portion of the air line  348  between the back flow preventers  349  and  351  being the higher of: (i) the air pressure in the one or more first air springs  106  of the first pneumatic circuit  102  and (ii) the one or more second air springs  110  of the second pneumatic circuit  104 . In the imbalanced state, the cross-flow air pressure sensor  118  that is in pneumatic communication with the air line  348  may output cross-flow pressure information that is indicative of the higher of: (i) the air pressure in the one or more first air springs  106  of the first pneumatic circuit  102  and (ii) the one or more second air springs  110  of the second pneumatic circuit  104 . In some aspects in which a computer  124  uses the cross-flow pressure information to provide braking control (e.g., to determine if system intervention is needed for anti-roll control) for the vehicle  10  and/or trailer  20 , the back flow preventers  349  and  351  may ensure that the cross-flow pressure information reflects the higher pressure in the imbalanced state. 
     In some aspects, as shown in  FIGS.  3 A,  3 B, and  3 D , the cross-flow air pressure sensor  118  may be separate from the computer  124 . In these aspects, the computer  124  may receive pressure information from the cross-flow air pressure sensor  118  (e.g., either directly from the cross-flow air pressure sensor  118  as an analog electrical signal or indirectly from the cross-flow air pressure sensor  118  as a digital electrical signal after being converted by an ADC). However, this is not required, and, in some alternative aspects, as shown in  FIGS.  3 C and  3 E , the computer  124  may include the cross-flow air pressure sensor  118 . In these aspects, computer  124  may receive the air line  342  or  352  as an input. 
     In some aspects, the system  100  may include a first air spring air pressure sensor  120 . In some aspects, the first air spring air pressure sensor  120  may be configured to output first air spring pressure information (e.g., an analog electrical signal) indicative of an air pressure within the one or more first air springs  106 . In some aspects, the first air spring air pressure sensor  120  may be inside an air spring of the one or more first air springs  106 . In some alternative aspects, the first pneumatic circuit  102  may include a fitting (e.g., a T-fitting) connected to the one or more first air springs  106 , and the first air spring air pressure sensor  120  may be configured to communicate pneumatically with the one or more first air springs  106  via the fitting. 
     In some aspects, the system  100  may include a second air spring air pressure sensor  122 . In some aspects, the second air spring air pressure sensor  122  may be configured to output second air spring pressure information (e.g., an analog electrical signal) indicative of an air pressure within the one or more second air springs  110 . In some aspects, the second air spring air pressure sensor  122  may be inside an air spring of the one or more second air springs  110 . In some alternative aspects, the second pneumatic circuit  104  may include a fitting (e.g., a T-fitting) connected to the one or more second air springs  110 , and the second air spring air pressure sensor  122  may be configured to communicate pneumatically with the one or more second air springs  110  via the fitting. 
     In some aspects, the one or more first air springs  106  and the one or more second air springs  110  may support a wheeled axle  12 ,  14 , or  15  (or a group of wheeled axles  14  and  15 ) of the vehicle  10  on a chassis of the vehicle  10 . In some aspects, the one or more first air springs  106  may be positioned on a first side of the axle  12 ,  14 , or  15  (or group of axles), and the one or more second air springs  110  may be positioned on a second side of the axle  12 ,  14 , or  15  (or group of axles), which is opposite the first side. For example, in some aspects, a pair of first air springs  106  may support an axle (e.g., axle  14  or  15 ) or group of axles (e.g., axles  14  and  15 ) on the first side, and a pair of second air springs  110  may support the axle or group of axles on the second side. 
     In some aspects, the first and second pneumatic circuits  102  and  104 , the cross-flow passage  116 , and the cross-flow air pressure sensor  118  may form a cross-flow set. In some aspects, the cross-flow set may include one or more additional components, such as, for example and without limitation, a first air spring air pressure sensor  120 , a second air spring air pressure sensor  122 , and/or the one or more air supply tanks  114 . In some aspects, the cross-flow set may be assigned to one axle or to a group of axle. In some aspects, one or more additional cross-flow sets may be assigned to different axles or groups of axles. 
       FIG.  2 B  illustrates an example of an aspect in which a cross-flow set is assigned to a group of axles (e.g., axles  14  and  15 ). As shown in  FIG.  2 B , the cross-flow set may include a pair of first air springs  106  that support axle  14  on a first side, a pair of first air springs  106  that support axle  15  on the first side, a pair of second air springs  110  that support axle  14  on a second side, a pair of second air springs  110  that support axle  15  on the second side, a first leveling valve  108  that adjusts independently the height of the axles  14  and  15  on the first side of the vehicle  10  (e.g., by increasing or decreasing air in the first air springs  106 ), a second leveling valve  112  that adjusts independently the height of the axles  14  and  15  on the second side of the vehicle  10  (e.g., by increasing or decreasing air in the second air springs  110 ), a cross-flow passage  116 , and a cross-flow air pressure sensor  118 . 
       FIG.  2 C  illustrates an example of an aspect in which a first cross-flow set is assigned to one axle (e.g., axle  12 ), and a second cross-flow set is assigned to a group of axles (e.g., axles  14  and  15 ). As shown in  FIG.  2 C , the first cross-flow set may include a pair of first air springs  106  that support axle  12  on a first side, a pair of second air springs  110  that support axle  12  on a second side, a first leveling valve  108  that adjusts independently the height of the axle  12  on the first side of the vehicle  10 , a second leveling valve  112  that adjusts independently the height of the axle  12  on the second side of the vehicle  10 , a cross-flow passage  116 , and a cross-flow air pressure sensor  118 . As shown in  FIG.  2 C , the second cross-flow set may include a pair of first air springs  106  that support axle  14  on a first side, a pair of first air springs  106  that support axle  15  on the first side, a pair of second air springs  110  that support axle  14  on a second side, a pair of second air springs  110  that support axle  15  on the second side, a first leveling valve  108  that adjusts independently the height of the axles  14  and  15  on the first side of the vehicle  10 , a second leveling valve  112  that adjusts independently the height of the axles  14  and  15  on the second side of the vehicle  10 , a cross-flow passage  116 , and a cross-flow air pressure sensor  118 . In some aspects having more than one cross-flow set (e.g., the aspect shown in  FIG.  2 C ), the computer  124  may receive cross-flow pressure information indicative of an air pressure within the cross-flow passage of each cross-flow set (e.g., cross-flow pressure information associated with axle  12  and cross-flow pressure information associated with the group of axles  14  and  15 ). 
     In some aspects, as shown in  FIG.  2 B , one or more of the axles (e.g., a steering axle  12 , which may be at the front of the vehicle  10 ) may be supported on the chassis of the vehicle  10  by mechanical springs  107  and  109  (instead of by air springs  106  and  110 ). In some aspects in which an axle is supported by mechanical springs  107  and  109 , as shown in  FIG.  2 B , the system  100  may include an axle load sensor  111 . In some aspects, the axle load sensor  111  may be attached to the center of the axle (e.g., axle  14 ). In some aspects, the axle load sensor  111  may be configured to output axle strain information indicative of the strain in the axle. In some aspects, the strain in the axle may, in turn, be indicative of the weight on the axle. 
     In some aspects, the system  100  may include a computer  124 . In some aspects, the computer  124  may include a processor and a non-transitory memory. In some aspects, the computer  124  may control the one or more aspects of the operation of the vehicle  10  and/or the trailer  20 . For example, the computer  124  may provide braking control for the vehicle  10  and/or trailer  20 , may provide load monitoring for the vehicle  10  and/or trailer  20 , and/or may provide air management for the vehicle  10  and/or trailer  20 . 
     In some aspects, the system  100  may include one or more analog to digital converters (ADCs)  126  configured to convert analog information into digital information. In some aspects, the system  100  may include one or more amplifiers  127  configured to amplify one or more analog signals. In some aspects, as shown in  FIG.  2 A , the computer  124  may include the one or more ADCs  126  and/or the one or more amplifiers  127 . However, this is not required, and, in some alternative aspects, the one or more ADCs  126  and/or the one or more amplifiers  127  may be elsewhere in the system  100 . 
     In some non-limiting aspects, the system  100  may include a brake pedal sensor  128 . In some aspects, the brake pedal sensor  128  may be configured to output a brake pedal pressure signal indicative of a pressure (e.g., mechanical pressure) applied to a brake pedal of the vehicle  10  (e.g., by the foot of a driver of the vehicle  10 ). 
     In some aspects, the system  100  may include one or more acceleration sensors  130 . In some aspects, the one or more acceleration sensors  130  may be configured to output acceleration information indicative of an acceleration of the vehicle  10 . In some aspects, the one or more acceleration sensors  130  may include one or more accelerometers. In some aspects, the acceleration information may include lateral acceleration information and/or longitudinal acceleration information. 
     In some aspects, the system  100  may include one or more speed sensors  132 . In some aspects, the one or more speed sensors  132  may be configured to output speed information indicative of a speed of the vehicle  10 . In some aspects, the one or more speed sensors  132  may include one or more wheel rotation sensors, and the speed information may include wheel rotational speed information for one or more wheels of the vehicle. In some aspects, one or more of the wheel rotation sensors may include, for example and without limitation, a tooth wheel mounted on the hub or rotor of the monitored wheel and a speed sensor installed with its end against the tooth wheel. In some aspects, a sensor clip may hold the speed sensor in place and against the tooth wheel. In some aspects, the one or more speed sensors  132  may be configured to continuously output speed information indicative of a speed of the vehicle  10 , which may be received by the computer  124 . 
     In some aspects, although not shown in  FIG.  2 A , the system  100  may include one or more steering angle sensors (SASs). In some aspects, the SASs may be configured to output steering angle information indicative of position of the steering wheel of the vehicle  10 . 
     In some aspects, the system  100  may include one or more first brakes  134  on the first side of the vehicle  10  and one or more second brakes  136  on the second side of the vehicle  10 . In some aspects, the first and second brakes  134  and  136  may be configured to be controlled in accordance with first and second brake application levels, respectively, calculated by the computer  124 . In some aspects, the first and second brake application levels may be brake application pressure levels. In some aspects, the first and second brake application levels may be different brake application levels. In some aspects, the computer  124  may apply the first and second brake application levels to the first and second brakes  134  and  136  via a valve block. In some aspects, the valve block may include one or more pressure modulator valves and/or one or more active braking valves (e.g., solenoid valves used for active braking) of the vehicle  10 . 
     In some aspects, the system  100  may include one or more displays  137 , one or more printers  138 , one or more communication units  139 , and one or more telematics units  141 . In some aspects, the one or more telematics units  141  may include one or more location systems (e.g., a global positioning system (GPS)) configured to output location information indicative of a location (e.g., a GPS location) of the vehicle  10  and/or trailer  20 . In some aspects, the one or more telematics units  141  may include one or more memories and/or one or more communication units. In some aspects, the one or more telematics units  141  may be configured to store and/or communicate tracking information about the vehicle  10  and/or trailer  20  (e.g., to a database server). In some aspects, the tracking information may include location information, speed and/or acceleration information, pressure information (e.g., cross-flow and/or air spring pressure information), axle strain information, brake pedal pressure signals, steering angle information, and/or weight information. In some aspects, the one or more telematics units  141  may receive the tracking information from the computer  124 . 
     In some aspects, the one or more displays  137  may be configured to display information received from the computer  124  (e.g., pressure or weight information). In some aspects, the one or more printers  138  may be configured to print information received from the computer  124  (e.g., pressure or weight information). In some aspects, the one or more communication units  139  may include one or more wireless communication units configured to communicate wirelessly using one or more wireless communication protocols (e.g., Bluetooth) with one or more remote devices (e.g., a smartphone or other display device). In some aspects, the one or more communication units  139  may be configured to communicate information received from the computer  124  (e.g., pressure or weight information) for display on a remote device. In some aspects, the one or more displays  137 , the one or more printers  138 , the one or more communication units  139 , and/or the one or more telematics units  141  may be mounted in a cab of the vehicle  10 . In some aspects, the system  100  may additionally or alternatively include one or more user interfaces through which a user may enter data and/or commands. 
     In some aspects, the one or more display  137 , the one or more telematics units  141 , and/or the one or more user interfaces may be part of an electronic control unit (ECU) of an on-board mass (OBM) system, one or more cross-flow air pressure sensors  118  (and an axle load sensor  111  if present in the system  100 ) may be part of one or more mass sensor units (MSUs) of the OBM system, and the computer  124  and one or more ADCs  126  may be part of the ECU and/or one or more MSUs of the OBM system. 
     In some aspects, the system  100  may include the trailer interface  143 . In some aspects, trailer interface  143  may include, for example and without limitation, a trailer plug and socket. In some aspects, the computer  124  of the vehicle  10  may receive information (e.g., pressure, speed, and/or acceleration information) from the trailer  20  via the trailer interface  143 . In some aspects, the trailer  20  may receive information (trailer brake application levels) from the computer  124  of the vehicle  10  via the trailer interface  143 . 
     In some aspects, the vehicle system  100  may include a trailer portion. In some aspects, the trailer portion of the vehicle system  100  may be a trailer system  200 .  FIG.  2 D  is a block diagram of a non-limiting aspect of the trailer system  200 . 
     In some aspects, the trailer system  200  may include a first trailer pneumatic circuit  202  and a second trailer pneumatic circuit  204 . The first trailer pneumatic circuit  202  may include one or more first trailer air springs  206  (e.g., one or more air bags) and a first trailer leveling valve  208 . The second trailer pneumatic circuit  204  may include one or more second trailer air springs  210  and a second trailer leveling valve  212 . In some aspects, the first trailer leveling valve  208  may be configured to adjust independently the height of a first side of the trailer  20  (e.g., by increasing or decreasing air in the one or more first trailer air springs  206 ), and the second trailer leveling valve  212  may be configured to adjust independently the height of a second side of the trailer  20  (e.g., by increasing or decreasing air in the one or more second trailer air springs  210 ). In some aspects, the first and second sides may be opposite sides of the trailer  20  (e.g., left and right sides). In some aspects, the trailer system  200  may include one or more trailer air supply tanks  214 . In some aspects, one or more supply lines may connect the one or more trailer air supply tanks  214  to the first trailer leveling valve  208 , and one or more supply lines may connect the one or more trailer air supply tanks  214  to the second trailer leveling valve  212 . 
     In some aspects, the first trailer leveling valve  208  may increase air in the one or more first trailer air springs  206  by supplying air from one or more of the one or more trailer air supply tanks  214 , and the second trailer leveling valve  212  may increase air in the one or more second trailer air springs  210  by supplying air from one or more of the one or more trailer air supply tanks  214 . In some aspects, the first trailer leveling valve  208  may decrease air in the one or more first trailer air springs  206  by purging air from the one or more first trailer air springs  206  (e.g., allowing air from the one or more first trailer air springs  206  out of an exhaust port), and the second trailer leveling valve  212  may decrease air in the one or more second trailer air springs  210  by purging air from the one or more second trailer air springs  210  (e.g., allowing air from the one or more second trailer air springs  210  out of an exhaust port). 
     In some aspects, as shown in  FIG.  2 D , the trailer system  200  may include one or more raise and lower valves (RLVs)  207 . In some aspects, the one or more RLVs  207  may allow an operator to manually vary the height of the chassis of the trailer  20  relative to the ground (e.g., to facilitate height adjustment of the trailer  20  at the loading dock). In some aspects, although the RLV  207  is shown as two units in  FIG.  2 D , the RLV  207  may be a single unit so that the operator may manually vary the height of the chassis of the trailer  20  relative to the ground on both sides of the trailer  20  using a single controller. In some alternative aspects, the trailer system  200  may include a first RLV  207  for manually varying the height of the chassis of the trailer on the first side of the trailer  20 , and the trailer system  200  may include a second MN  207  for manually varying the height of the chassis of the trailer  20  on the second side of the trailer  20 . 
     In some aspects, the one or more RLVs  207  may be connected by air lines to the first trailer leveling valve  208 , the one or more first trailer air springs  206 , the second trailer leveling valve  212 , and the one or more second trailer air springs  210 . In some aspects, although not shown in  FIG.  2 D , one or more supply lines may supply air from the one or more trailer air supply tanks  214  to the one or more RLVs  207  (e.g., for increasing the height of the chassis of the trailer  20  relative to the ground). In some aspects, when set in a neutral or drive mode (e.g., while the trailer  20  is traveling), the one or more RLVs  207  may enable pneumatic communication between the first trailer leveling valve  208  and the one or more first trailer air springs  206  and pneumatic communication between the second trailer leveling valve  212  and the one or more second trailer air springs  210  so that the first and second trailer leveling valves  208  and  212  perform automatic height control by controlling the amounts of air in the first and second trailer air springs  206  and  210 , respectively. 
     In some aspects, the operator may use the one or more RLVs  207  to manually control the height of the chassis of the trailer  20  relative to the ground when first and second trailer leveling valves  208  and are not performing automatic height control (e.g., while the trailer  20  is stationary, such as at a loading dock). In some aspects, when the operator is manually controlling the height of the chassis of the trailer  20  relative to the ground, the one or more RLVs  207  may be set to one of a stop mode, a raise mode, and a lower mode. In some aspects, in each of the stop, raise, and lower modes, the one or more RLVs  207  may prevent pneumatic communication between the first trailer leveling valve  208  and the one or more first trailer air springs  206  and prevent pneumatic communication between the second trailer leveling valve  212  and the one or more second trailer air springs  210 . In some aspects, when set in the stop mode, the one or more RLVs  207  may prevent all pneumatic communication through the one or more RLVs  207  so that height of the chassis of the trailer  20  relative to the ground does not change. In some aspects, when set in the raise mode, the one or more RLVs  207  may enable pneumatic communication between the one or more trailer air supply tanks  214  and the one or more first trailer air springs  206  and pneumatic communication between the one or more trailer air supply tanks  214  and the one or more second trailer air springs  210  increase the amount of air in the first and second trailer air springs  206  and  210  and, thereby, increase the height of the chassis of the trailer  20 . In some aspects, when set in the lower mode, the one or more RLVs  207  may vent air from the one or more first trailer air springs  206  and the one or more second trailer air springs  210  to decrease the amount of air in the first and second trailer air springs  206  and  210  and, thereby, decrease the height of the chassis of the trailer  20 . In some aspects, in all of the RLV modes, the one or more RLVs  207  may prevent the one or more first trailer air springs  206  from communicating pneumatically with the second trailer leveling valve  212  and the one or more second trailer air springs  210  and may prevent the first trailer leveling valve  208  from communicating pneumatically with the second trailer leveling valve  212  and the one or more second trailer air springs  210 . 
     However, one or more RLVs  207  are not required, and, in some alternative aspects, the trailer system  200  may not include an RLV, one or more air lines may connect directly the first trailer leveling valve  208  and the one or more first trailer air springs  206 , and one or more air lines may connect directly the second trailer leveling valve  212  and the one or more second trailer air springs  210 . In some aspects, the trailer system  200  may include one or more dump valves. In some aspects, one or more air lines may connect and provide pneumatic communication (i) between a dump port of the first trailer leveling valve  208  to the dump valve and (ii) between a dump port of the second trailer leveling valve  212  to the dump valve. In some aspects, an air line may connect and provide pneumatic communication between the one or more trailer air supply tanks  214  and the dump valve. In some aspects, the dump valve may be used to dump (manually or automatically) air from the trailer air springs  206  and  210   
     In some aspects, the trailer system  200  may include one or more pressure protection valves (PPVs), which may protect the trailer system  200  in the event of a leak or failure within the trailer system  200 . For example, as shown in  FIG.  2 D , the trailer system  200  may include a first PPV  215  between the one or more trailer air supply tanks  214  and the first trailer leveling valve  208  and a second PPV  217  between the one or more trailer air supply tanks  214  and the second trailer leveling valve  212 . In some aspects, a supply line may connect the one or more trailer air supply tanks  214  to first PPV  215 , and a supply line may connect the first PPV  215  and the first trailer leveling valve  208 . In some aspects, a supply line may connect the one or more trailer air supply tanks  214  to the second PPV  217 , and a supply line may connect the second PPV  217  and the second trailer leveling valve  212 . In some alternative aspects, the first and second PPVs  215  and  217  may be integrated into a single PPV unit (e.g., a dual PPV including an inlet, two outlets, and an exhaust), the one or more trailer air supply tanks  214  may be connected to an inlet of the integrated PPV unit, a first air line may connect a first outlet of the integrated PPV unit and the first trailer leveling valve  208 , and a second air line may connect a second outlet of the integrated PPV unit and the second trailer leveling valve  212 . However, the one or more PPVs are not required, and, in some alternative aspects, a supply line may connect the one or more trailer air supply tanks  214  directly to the first trailer leveling valve  208 , and a supply line may connect the one or more trailer air supply tanks  214  directly to the second trailer leveling valve  212 . 
     In some aspects, the air lines of the first and second trailer pneumatic circuits  202  and  204  may be configured to supply equal volumes of air to maintain symmetry within the trailer pneumatic circuits  202  and  204  on both sides of the trailer  20 . In some aspects, the air lines of the first and second trailer pneumatic circuits  202  and  204  may include (i) an air line connecting the first trailer leveling valve  208  and the RLV  207  and one or more air lines connecting the RLV  207  and the one or more first trailer air springs  206  (or one or more air lines connecting the first trailer leveling valve  208  and the one or more first trailer air springs  206  if there is no RLV  207 ), (ii) an air line connecting the second trailer leveling valve  212  and the RLV  207  and one or more air lines connecting the RLV  207  and the one or more second trailer air springs  210  (or one or more air lines connecting the second trailer leveling valve  212  and the one or more second trailer air springs  210  if there is no RLV  207 ), (iii) a supply line connecting the one or more trailer air supply tanks  214  to a PPV (e.g., the first PPV  215 ) and a supply line may connect the PPV and the first trailer leveling valve  208  (or one or more supply lines connecting the one or more trailer air supply tanks  214  and the first trailer leveling valve  208  if there is no PPV), and/or (iv) a supply line connecting the one or more trailer air supply tanks  214  to a PPV (e.g., the second PPV  217 ) and a supply line may connect the PPV and the second trailer leveling valve  212  (or one or more supply lines connecting the one or more trailer air supply tanks  214  and the second trailer leveling valve  212  if there is no PPV). In some aspects, one or more of the air lines of the first trailer pneumatic circuit  202  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as one or more of the corresponding air lines of the second trailer pneumatic circuit  204 . 
     The present invention is not limited to the trailer  20  having the particular number of axle(s)  16 ,  18 ,  19 , and/or  21 , trailer air springs  206  and  210 , air lines/hoses, and/or trailer air supply tank(s)  214  that are shown in the drawings, as these elements vary depending on the type of trailer that is used as would be immediately clear to a person skilled in the art. In some aspects, as shown in  FIG.  2 D , the first trailer pneumatic circuit  202  and a second trailer pneumatic circuit  204  may be supplied air by one or more common trailer air supply tanks  214 . However, this is not required, and, in some alternative aspects, one or more trailer air supply tanks may be dedicated to the first trailer pneumatic circuit  202 , and one or more different trailer air supply tanks may be dedicated to the second trailer pneumatic circuit  204 . 
     In some aspects, the trailer system  200  may include a trailer cross-flow passage  216  that connects the first trailer leveling valve  208  with the second trailer leveling valve  212 . The first and second trailer leveling valves  208  and  212  may establish pneumatic communication between the first and second trailer pneumatic circuits  202  and  204  when neither the first trailer leveling valve  208  is adjusting independently the height of the first side of the trailer  20  nor the second trailer leveling valve  212  is adjusting independently the height of the second side of the trailer  20  (e.g., when (i) the first trailer leveling valve  208  is neither supplying air to nor purging air from the one or more first trailer air springs  206  and (ii) the second trailer leveling valve  212  is neither supplying air to nor purging air from the one or more trailer second air springs  210 ). In some aspects, the trailer cross-flow passage  216  is not directly connected to the one or more trailer air supply tanks  214  (or to a supply line connected to a trailer air supply tank  214 ). In some aspects, the pneumatic communication between the first and second trailer pneumatic circuits  202  and  204  via the trailer cross-flow passage  216  may equalize air pressure between the one or more first trailer air springs  206  and the one or more second trailer air springs  210 . As a result, the first and second trailer leveling valves  208  and  212  may link together the first and second trailer pneumatic circuits  202  and  204  as a common circuit when neither the first trailer leveling valve  208  is adjusting independently the height of the first side of the trailer  20  nor the second trailer leveling valve  212  is adjusting independently the height of the second side of the trailer  20 . 
     Accordingly, during a shift in the center of gravity of the trailer  20  (e.g., when the trailer  20  is negotiating a sharp turn and/or is on uneven ground), one of the first and second trailer leveling valves  208  and  212  may supply air to the one of the first and second trailer air springs  206  and  210  that have been contracted from the weight shift of the trailer  20  while the other one of the first and second trailer leveling valves  208  and  212  purges air from the other one of the first and second trailer air springs  206  and  210  that have been extended from the weight shift of the trailer  20  without any cross-flow between the first and second trailer pneumatic circuits  202  and  204 . In this state, the first and second trailer leveling valves  208  and  212  may overcompensate for the dynamic weight shift of the trailer  20  by either supplying too much air to one of the first and second trailer air springs  206  and  210  or removing too much air from the other of the first and second trailer air springs  206  and  210 , resulting in a slight pressure difference between the first and second trailer air springs  206  and  210 . When the shift in the center of gravity of the trailer  20  ends, if not for the first and second trailer leveling valves  208  and  212  establishing pneumatic communication between the first and second trailer pneumatic circuits  202  and  204 , the slight pressure difference would keep the trailer  20  in an uneven state. However, because the first and second trailer leveling valves  208  and  212  establish pneumatic communication between the first and second trailer pneumatic circuits  202  and  204  when neither the first trailer leveling valve  208  is adjusting independently the height of the first side of the trailer  20  nor the second trailer leveling valve  212  is adjusting independently the height of the second side of the trailer  20 , the slight pressure difference between the first and second trailer air springs s06 and s10 is eliminated as air passes via the trailer cross-flow passage  216  from the one of the first and second trailer air springs  206  and  210  at higher pressure to the other of the first and second trailer air springs  206  and  210  at lower pressure, thereby reaching an equilibrium state. 
     In some aspects, the trailer system  200  may include a trailer cross-flow air pressure sensor  218 . In some aspects, the trailer cross-flow air pressure sensor  218  may be configured to output trailer cross-flow pressure information (e.g., an analog electrical signal) indicative of an air pressure within the trailer cross-flow passage  216 . In some aspects, similar to the setup shown in  FIG.  3 A , the trailer cross-flow air pressure sensor  218  may be inside the trailer cross-flow passage  216 . In some alternative aspects, similar to the setup shown in  FIGS.  3 B and  3 C , the trailer system  200  may include a fitting (e.g., a T-fitting) connected to the trailer cross-flow passage  216 , and the trailer cross-flow air pressure sensor  218  may be configured to communicate pneumatically with the trailer cross-flow passage  216  via the fitting. In some aspects, one or more air lines may connect the trailer cross-flow air pressure sensor  218  to the fitting for pneumatic communication between the trailer cross-flow air pressure sensor  218  and the trailer cross-flow passage  216 . 
     In some alternative aspects, similar to the setup shown in  FIGS.  3 D and  3 E , the trailer system  200  may include a first air line connecting the one or more first trailer air springs  206  of the first trailer pneumatic circuit  202  and the one or more second trailer air springs  210  of the second trailer pneumatic circuit  204 . In some aspects, the first air line may have a smaller diameter than the trailer cross-flow passage  216 . In some aspects, a fitting (e.g., a T-fitting) may be connected to the first air line, and the trailer cross-flow air pressure sensor  218  may be configured to communicate pneumatically with the first air line via the fitting. In some aspects, the first air line may include back flow preventers on opposite sides of the fitting. In some aspects, one or more second air lines may connect the trailer cross-flow air pressure sensor  218  to the fitting for pneumatic communication between the trailer cross-flow air pressure sensor  218  and the first air line. In some aspects, a residual pressure purge device (e.g., a purge solenoid) may be included to vent residual pressure from the first air line connecting the one or more first trailer air springs  206  of the first trailer pneumatic circuit  202  and the one or more second trailer air springs  210  of the second trailer pneumatic circuit  204 . 
     In some aspects, similar to the setup shown in  FIGS.  3 A,  3 B, and  3 D , the trailer cross-flow air pressure sensor  218  may be separate from the computer  124 . In these aspects, the computer  124  may receive pressure information from the trailer cross-flow air pressure sensor  218  (e.g., either directly from the trailer cross-flow air pressure sensor  218  as an analog electrical signal or indirectly from the trailer cross-flow air pressure sensor  218  as a digital electrical signal after being converted by an ADC). However, this is not required, and, in some alternative aspects, similar to the setup shown in  FIGS.  3 C and  3 E , the computer  124  (e.g., a portion of the computer  124  in the trailer  20 ) may include the trailer cross-flow air pressure sensor  218 . In these aspects, computer  124  (e.g., a portion of the computer  124  in the trailer  20 ) may receive an air line as an input. 
     In some aspects, the trailer system  200  may include a first trailer air spring air pressure sensor  220 . In some aspects, the first trailer air spring air pressure sensor  220  may be configured to output first trailer air spring pressure information (e.g., an analog electrical signal) indicative of an air pressure within the one or more first trailer air springs  206 . In some aspects, the first trailer air spring air pressure sensor  220  may be inside an air spring of the one or more first trailer air springs  206 . In some alternative aspects, the first trailer pneumatic circuit  202  may include a fitting (e.g., a T-fitting) connected to the one or more first trailer air springs  206 , and the first trailer air spring air pressure sensor  220  may be configured to communicate pneumatically with the one or more first trailer air springs  206  via the fitting. 
     In some aspects, the trailer system  200  may include a second trailer air spring air pressure sensor  222 . In some aspects, the second trailer air spring air pressure sensor  222  may be configured to output second trailer air spring pressure information (e.g., an analog electrical signal) indicative of an air pressure within the one or more second trailer air springs  210 . In some aspects, the second trailer air spring air pressure sensor  222  may be inside an air spring of the one or more second trailer air springs  210 . In some alternative aspects, the second trailer pneumatic circuit  204  may include a fitting (e.g., a T-fitting) connected to the one or more second trailer air springs  210 , and the second trailer air spring air pressure sensor  222  may be configured to communicate pneumatically with the one or more second trailer air springs  210  via the fitting. 
     In some aspects, the trailer system  200  may include one or more analog to digital converters (ADCs) (not shown) configured to convert analog information into digital information. In some aspects, the trailer system  200  may include one or more amplifiers (not shown) configured to amplify one or more analog signals. 
     In some aspects, the trailer system  200  may include one or more trailer acceleration sensors  230 . In some aspects, the one or more trailer acceleration sensors  230  may be configured to output trailer acceleration information indicative of an acceleration of the trailer  20 . In some aspects, the one or more trailer acceleration sensors  230  may include one or more accelerometers. In some aspects, the trailer acceleration information may include lateral acceleration information and/or longitudinal acceleration information. 
     In some aspects, the trailer system  200  may include one or more trailer speed sensors  232 . In some aspects, the one or more trailer speed sensors  232  may be configured to output trailer speed information indicative of a speed of the trailer  20 . In some aspects, the one or more trailer speed sensors  232  may include one or more wheel rotation sensors, and the speed information may include wheel rotational speed information for one or more wheels of the trailer  20 . 
     In some aspects, the trailer system  200  may include one or more first trailer brakes  234  on the first side of the trailer  20  and one or more second trailer brakes  236  on the second side of the trailer  20 . In some aspects, the first and second trailer brakes  234  and  236  may be configured to be controlled in accordance with first and second trailer brake application levels, respectively, calculated by the computer  124  of the vehicle system  100 . In some aspects, the first and second trailer brake application levels may be different brake application levels. In some aspects, the first trailer brake application level applied to the first trailer brake  234  may be different than the first vehicle brake application applied to the first brake  134 . In some aspects, the second trailer brake application level applied to the second trailer brake  236  may be different than the second vehicle brake application applied to the second brake  136 . In some aspects, the computer  124  may apply the first and second trailer brake application levels to the first and second trailer brakes  234  and  236  via a valve block. In some aspects, the valve block may include one or more pressure modulator valves and/or one or more active braking valves (e.g., solenoid valves used for active braking) of the trailer  20 . 
     In some aspects, the one or more first trailer air springs  206  and the one or more second trailer air springs  210  may support a wheeled axle  16 ,  18 ,  19 , or  21  (or a group of wheeled axles) of the trailer  20  on a chassis of the trailer  20 . In some aspects, the one or more first trailer air springs  206  may be positioned on a first side of the axle  16 ,  18 ,  19 , or  21  (or a group of axles), and the one or more second trailer air springs  210  may be positioned on a second side of the axle  16 ,  18 ,  19 , or  21  (or a group of axles), which is opposite the first side. For example, in some aspects, a pair of first trailer air springs  206  may support an axle (e.g., axle  16 ,  18 ,  19 , or  21 ) or group of axles (e.g., axles  18  and  19 ) on the first side, and a pair of second trailer air springs  210  may support the axle or group of axles on the second side. 
     In some aspects, the first and second trailer pneumatic circuits  202  and  204 , the trailer cross-flow passage  216 , and the trailer cross-flow air pressure sensor  218  may form a trailer cross-flow set. In some aspects, the trailer cross-flow set may include one or more additional components, such as, for example and without limitation, a first trailer air spring air pressure sensor  220 , a second trailer air spring air pressure sensor  222 , and/or one or more trailer air supply tanks  214 . In some aspects, the trailer cross-flow set may be assigned to one axle or to a group of axle. In some aspects, one or more additional trailer cross-flow sets may be assigned to different axles or groups of axles. 
       FIG.  2 E  illustrates an example of an aspect in which a trailer cross-flow set is assigned to a group of axles (e.g., axles  18  and  19 ). As shown in  FIG.  2 E , the trailer cross-flow set may include a pair of first trailer air springs  206  that support axle  18  on a first side, a pair of first trailer air springs  206  that support axle  19  on the first side, a pair of second trailer air springs  210  that support axle  18  on a second side, a pair of second trailer air springs  210  that support axle  19  on the second side, a first trailer leveling valve  208  that adjusts independently the height of the axles  18  and  19  on the first side of the trailer  20  (e.g., by increasing or decreasing air in the first trailer air springs  206 ), a second trailer leveling valve  212  that adjusts independently the height of the axles  18  and  19  on the second side of the trailer  20  (e.g., by increasing or decreasing air in the second trailer air springs  210 ), a trailer cross-flow passage  216 , and a trailer cross-flow air pressure sensor  218 . 
     In some aspects, as shown in  FIG.  2 E , one or more of the axles (e.g., a steering axle  16 , which may be at the front of the trailer  20 ) may be supported on the chassis of the trailer  20  by mechanical springs  207  and  209  (instead of by trailer air springs  206  and  210 ). In some aspects in which an axle is supported by mechanical springs  207  and  209 , as shown in  FIG.  2 E , the trailer system  200  may include a trailer axle load sensor  211 . In some aspects, the trailer axle load sensor  211  may be attached to the center of the axle (e.g., axle  16 ). In some aspects, the trailer axle load sensor  211  may be configured to output trailer axle strain information indicative of the strain in the axle. In some aspects, the strain in the axle may, in turn, be indicative of the weight on the axle. 
       FIG.  2 F  illustrates an example of an aspect in which a first trailer cross-flow set is assigned to one axle (e.g., axle  18 ), a second trailer cross-flow set is assigned to another axle (e.g., axle  19 ), and a third trailer cross-flow set is assigned to yet another axle (e.g., axle  21 ). The first, second, and third cross-flow sets shown in  FIG.  2 F  may be used, for example and without limitation, a trailer  20  that is a triple spread axle trailer. As shown in  FIG.  2 F , the third trailer cross-flow set may include a pair of first trailer air springs  206  that support axle  21  on a first side, a pair of second trailer air springs  210  that support axle  21  on a second side, a first trailer leveling valve  208  that adjusts independently the height of the axle  21  on the first side of the trailer  20 , a second trailer leveling valve  212  that adjusts independently the height of the axle  21  on the second side of the trailer  20 , a trailer cross-flow passage  216 , and a trailer cross-flow air pressure sensor  218 . 
     In some aspects having more than one trailer cross-flow set (e.g., the aspect shown in  FIG.  2 F ), the computer  124  may receive trailer cross-flow pressure information indicative of an air pressure within the trailer cross-flow passage  216  of each trailer cross-flow set (e.g., two or more of trailer cross-flow pressure information associated with axle  18 , trailer cross-flow pressure information associated with axle  19 , and trailer cross-flow pressure information associated with axle  21 ). In some aspects, the one or more trailer cross-flow air pressure sensors  218  (and the trailer axle load sensor  211  if present) may be part of one or more MSUs of the on-board mass (OBM) system. 
       FIGS.  2 G and  2 O  illustrate examples of aspects in which a trailer cross-flow set is assigned to a group of axles (e.g., axles  18 ,  19 , and  21 ) for a trailer  20  (e.g., a triaxle trailer). The provided figures are not intended to be limiting and the present disclosure further includes analogous systems with various numbers of axles, e.g., 1, 2, 3, 4, 5, or more axles. As shown in  FIGS.  2 G and  2 O , the trailer cross-flow set may include a first trailer air spring  206   a  that supports an axle  18  on a first side, a first trailer air spring  206   b  that supports an axle  19  on the first side, a first trailer air spring  206   c  that supports an axle  21  on the first side, a second trailer air spring  210   a  that supports the axle  18  on the second side, a second trailer air spring  210   b  that supports the axle  19  on the second side, a second trailer air spring  210   c  that supports the axle  21  on the second side, a first trailer leveling valve  208  that adjusts independently the height of the axles  18 ,  19 , and  21  on the first side of the trailer  20  (e.g., by increasing or decreasing air in the first trailer air springs  206   a - c ), a second trailer leveling valve  212  that adjusts independently the height of the axles  18 ,  19 , and  21  on the second side of the trailer  20  (e.g., by increasing or decreasing air in the second trailer air springs  210   a - c ), a trailer cross-flow passage  216 , and a trailer cross-flow air pressure sensor  218 . In some aspects, as shown in  FIGS.  1 B,  2 G, and  2 O , the axle  19  may be a central axle located between the axles  18  and  21 . In some aspects, as shown in  FIGS.  2 G and  2 O , the first trailer air spring  206   b  that supports the central axle  19  may be a central first trailer air spring  206   b  located between the first trailer air spring  206   a  and  206   c  that support the axles  18  and  21 , respectively. In some aspects, as shown in  FIGS.  2 G and  2 O , the second trailer air spring  210   b  that supports the central axle  19  may be a central second trailer air spring  210   b  located between the second trailer air spring  210   a  and  210   c  that support the axles  18  and  21 , respectively. 
     In some aspects, similar to the example illustrated in  FIG.  2 E , air lines may connect the first trailer leveling valve  208  (either directly or through a RLV) to each of the first trailer air springs  206   a - c , and air lines may connect the second trailer leveling valve  212  (either directly or through an RLV) to each of the second trailer air springs  210   a - c . In some alternative aspects, as shown in  FIG.  2 G , an air line may connect the first trailer leveling valve  208  (either directly or through an RLV) to the central first trailer air spring  206   b  that supports the axle  19 , an air line  225  may connect and provide pneumatic communication between the central first trailer air spring  206   b  and the first trailer air spring  206   a  that supports the axle  18 , and an air line  224  may connect and provide pneumatic communication between the central first trailer air spring  206   b  and the first trailer air spring  206   c  that supports the axle  21 . In these alternative aspects, the air lines  224  and  225  may enable front-to-back (and back-to-front) air flow between the first trailer air springs  206   a - c  with the central first trailer air spring  206   b  acting as an air reservoir between the first trailer air springs  206   a  and  206   c.    
     In these alternative aspects, as shown in  FIG.  2 G , an air line may connect the second trailer leveling valve  212  (either directly or through an RLV) to the central second trailer air spring  210   b  that supports the axle  19 , an air line  227  may connect and provide pneumatic communication between the central second trailer air spring  210   b  and the second trailer air spring  210   a  that supports the axle  18 , and an air line  226  may connect and provide pneumatic communication between the central second trailer air spring  210   b  and the second trailer air spring  210   c  that supports the axle  21 . In these alternative aspects, the air lines  226  and  227  may enable front-to-back (and back-to-front) air flow between the second trailer air springs  210   a - c  with the central second trailer air spring  210   b  acting as an air reservoir between the second trailer air springs  210   a  and  210   c.    
     In some other alternative aspects, as shown in  FIG.  2 O , a first air line may connect the first trailer leveling valve  208  (either directly or through an RLV) to the air line  224  that connects and provides pneumatic communication between the central first trailer air spring  206   b  and the first trailer air spring  206   c , a second air line may connect the first trailer leveling valve  208  (either directly or through an RLV) to the air line  225  that connects and provides pneumatic communication between the central first trailer air spring  206   b  and the first trailer air spring  206   a , a third air line may connect the second trailer leveling valve  212  (either directly or through an RLV) to the air line  226  that connects and provides pneumatic communication between the central second trailer air spring  210   b  and the second trailer air spring  210   c , and a fourth air line may connect the second trailer leveling valve  212  (either directly or through an RLV) to the air line  227  that connects and provides pneumatic communication between the central second trailer air spring  210   b  and the second trailer air spring  210   a . In some of these alternative aspects, the first, second, third, and fourth air lines may be connected to the air lines  224 ,  225 ,  226 , and  227 , respectively, via a fitting (e.g., a T-fitting). In some of these alternative aspects, the first, second, third, and fourth air lines may be connected to the air lines  224 ,  225 ,  226 , and  227 , respectively, at the mid-points of the air lines  224 ,  225 ,  226 , and  227 , respectively. 
       FIGS.  2 H- 2 N,  2 P, and  2 Q  illustrate pneumatic circuits (e.g., first and second trailer pneumatic circuits  202  and  204 ) of a trailer system  200  for a trailer  20  (e.g., a triaxle trailer) according to various aspects. In some aspects, the air lines and/or supply lines illustrated in  FIGS.  2 H- 2 N,  2 P, and  2 Q  may be nylon hoses (e.g., nylon hoses having a ½ inch diameter). In some aspects, the air lines of the trailer system  200  may be configured to supply equal volumes of air to maintain symmetry within the pneumatic circuits on both sides of the trailer  20 . In some aspects, the air line  225  between the central first trailer air spring  206   b  and the first trailer air spring  206   a  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  227  between the central second trailer air spring  210   b  and the second trailer air spring  210   a . In some aspects, the air line  225  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  224  between the central first trailer air spring  206   b  and the first trailer air spring  206   c . In some aspects, the air line  224  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  226  between the central second trailer air spring  210   b  and the second trailer air spring  210   c . In some aspects, all of the air lines  224 - 227  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length. 
     In some aspects, as shown in  FIGS.  2 H- 2 J and  2 P , the trailer system  200  may include an RLV  207 , an air line  245  that connects and provides pneumatic communication between the first trailer leveling valve  208  and the RLV  207 , and an air line  246  that connects and provides pneumatic communication between the second trailer leveling valve  212  and the RLV  207 . In some aspects, as shown in  FIGS.  2 H- 2 J , the trailer system  200  may include an air line  247  that connects and provides pneumatic communication between the RLV  207  and the central first trailer air spring  206   b , an air line  248  that connects and provides pneumatic communication between the RLV  207  and the central second trailer air spring  210   b , and an air line (not shown) that connects and provides pneumatic communication between the one or more trailer air supply tanks  214  and the RLV  207 . In some aspects, the air line  245  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  246 . In some aspects, the air line  247  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  248 . 
     In some alternative aspects, as shown in  FIG.  2 K , the trailer system  200  may include a first RLV  207   a  for the first side of the trailer  20  and a second RLV  207   b  for the second side of the trailer  20 , the air line  245  may connect and provide pneumatic communication between the first trailer leveling valve  208  and the first RLV  207   a , the air line  246  may connect and provide pneumatic communication between the second trailer leveling valve  212  and the second RLV  207   b , the air line  247  may connect and provide pneumatic communication between the first RLV  207   a  and the central first trailer air spring  206   b , the air line  248  may connect and provide pneumatic communication between the second RLV  207   b  and the central second trailer air spring  210   b , an air line (not shown) may connect and provide pneumatic communication between the one or more trailer air supply tanks  214  and the first RLV  207   a , and an air line (not shown) may connect and provide pneumatic communication between the one or more trailer air supply tanks  214  and the second RLV  207   b . In some aspects, the air line  245  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  246 . In some aspects, the air line  247  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  248 . 
     In some alternative aspects, as shown in  FIG.  2 P , the trailer system  200  may include an air line  247   a  that connects and provides pneumatic communication between the RLV  207  and the air line  224 , an air line  247   b  that connects and provides pneumatic communication between the RLV  207  and the air line  225 , an air line  248   a  that connects and provides pneumatic communication between the RLV  207  and the air line  226 , an air line  248   b  that connects and provides pneumatic communication between the RLV  207  and the air line  227 , and an air line (not shown) that connects and provides pneumatic communication between the one or more trailer air supply tanks  214  and the RLV  207 . In some aspects, the RLV  207  may include only one port for connection to the first trailer air springs  206  and only one port for connection to the second trailer air springs  210 , and, in this case, T-junctions may be used to connect the air lines  247   a ,  247   b ,  248   a , and  248   b  to the RLV  207 . That is, in some aspects, the air lines  247   a  and  247   b  may connect to a T-junction at the RLV port for connection to the first trailer air springs  206 , and the air lines  248   a  and  248   b  for connection to the second trailer air springs  210 . In some aspects, the air line  247   a  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  248   a . In some aspects, the air line  247   b  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  248   b . In some aspects, the air line  247   a  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  247   b . In some aspects, the air line  248   a  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  248   b.    
     In some other alternative aspects, as shown in  FIGS.  2 L,  2 M,  2 N, and  2 P , the trailer system  200  may not include an RLV. In some aspects, as shown in  FIGS.  2 L,  2 M, and  2 N , and the trailer system  200  may include an air line  255  that connects and provides pneumatic communication between the first trailer leveling valve  208  and the central first trailer air spring  206   b  and an air line  256  that connects and provides pneumatic communication between the second trailer leveling valve  212  and the central second trailer air spring  210   b . In some aspects, the air line  255  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  256 . 
     In some alternative aspects, as shown in  FIG.  2 P , the trailer system  200  may include an air line  255   a  that connects and provides pneumatic communication between the first trailer leveling valve  208  and the air line  224 , an air line  255   b  that connects and provides pneumatic communication between the first trailer leveling valve  208  and the air line  225 , an air line  256   a  that connects and provides pneumatic communication between the second trailer leveling valve  212  and the air line  226 , an air line  256   b  that connects and provides pneumatic communication between the second trailer leveling valve  212  and the air line  227 , and an air line (not shown) that connects and provides pneumatic communication between the one or more trailer air supply tanks  214  and the RLV  207 . In some aspects, the air line  255   a  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  256   a . In some aspects, the air line  255   b  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  256   b . In some aspects, the air line  255   a  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  255   b . In some aspects, the air line  256   a  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the air line  256   b.    
     In some aspects, as shown in  FIG.  2 H , the trailer system  200  may include first and second PPVs  215  and  217 , a supply line  241  that connects and provides pneumatic communication between the one or more trailer air supply tanks  214  and the first PPV  215 , a supply line  242  that connects and provides pneumatic communication between the one or more trailer air supply tanks  214  and the second PPV  217 , a supply line  243  that connects and provides pneumatic communication between the first PPV  215  and the first trailer leveling valve  208 , and a supply line  244  that connects and provides pneumatic communication between the second PPV  217  and the second trailer leveling valve  212 . In some aspects, the supply line  241  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the supply line  242 . In some aspects, the supply line  243  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the supply line  244 . 
     In some alternative aspects, as shown in  FIGS.  2 I,  2 M,  2 N,  2 P, and  2 Q , the trailer system  200  may include an integrated PPV unit  219  for both the first and second trailer leveling valves  208  and  212 . In some aspects, the integrated PPV unit  219  may be a dual PPV including an inlet, two outlets, and an exhaust. In some aspects, the integrated PPV unit  219  may be easier to install than separate PPVs  215  and  217 . In some aspects, as shown in  FIGS.  2 I,  2 M, and  2 N , a supply line  251  that connects and provides pneumatic communication between the one or more trailer air supply tanks  214  and an inlet of the integrated PPV unit  219 , a supply line  249  that connects and provides pneumatic communication between the a first outlet of the integrated PPV unit  219  and the first trailer leveling valve  208 , and a supply line  250  that connects and provides pneumatic communication between a second outlet of the integrated PPV unit  219  and the second trailer leveling valve  212 . In some aspects, the supply line  249  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the supply line  250 . 
     In some alternative aspects, as shown in  FIGS.  2 J,  2 K, and  2 L , the trailer system  200  may not include a PPV, a supply line  253  may connect and provide pneumatic communication between the one or more trailer air supply tanks  214  and the first trailer leveling valve  208 , and a supply line  254  may connect and provide pneumatic communication between the one or more trailer air supply tanks  214  and the second trailer leveling valve  212 . In some aspects, the supply line  253  may be of substantially the same (e.g., within ±10% or ±5% or ±2% or ±1%) or equal diameter and/or length as the supply line  254 . 
     In some aspects, as shown in  FIG.  2 M , the trailer system  200  may include a dump valve  258 . In some aspects, one or more air lines  257  may connect and provide pneumatic communication (i) between a dump port of the first trailer leveling valve  208  to the dump valve  258  and (ii) between a dump port of the second trailer leveling valve  212  to the dump valve  258 . In some aspects, an air line (not shown) may connect and provide pneumatic communication between the one or more trailer air supply tanks  214  and the dump valve  258 . In some aspects, the dump valve  258  may be used to dump (manually or automatically) air from the trailer air springs  206  and  210 . However, the dump valve  258  is not required, and, in some alternative aspects, as shown in  FIGS.  2 H- 2 L,  2 N,  2 P, and  2 Q , the trailer system  200  may not include a dump valve. 
     In some aspects, as described above, the vehicle system  100  and/or trailer system  200  may integrate height control, load monitoring, and/or braking control.  FIG.  2 R  illustrates a trailer system  200  integrating height control, load monitoring, and braking control according to some non-limiting aspects. Although an integrated trailer system  200  is illustrated in  FIG.  2 R , the vehicle system  100  may be integrated in a similar fashion. 
     In some aspects, as shown in  FIG.  2 R , the trailer  20  may be a triaxle trailer including axles  18 ,  19 , and  21 . However, three axles are not required, and, in alternative aspects, the trailer  20  may include a different number of axles (e.g., 1, 2, 4, 5, or more axles). In some aspects, similar to  FIG.  2 O , a trailer cross-flow set may be assigned to a group of axles (e.g., axles  18 ,  19 , and  21 ) for the trailer  20 . As shown in  FIG.  2 R , the trailer cross-flow set may include a first trailer air spring  206   a  that supports an axle  18  on a first side, a first trailer air spring  206   b  that supports an axle  19  on the first side, a first trailer air spring  206   c  that supports an axle  21  on the first side, a second trailer air spring  210   a  that supports the axle  18  on the second side, a second trailer air spring  210   b  that supports the axle  19  on the second side, a second trailer air spring  210   c  that supports the axle  21  on the second side, a first trailer leveling valve  208  that adjusts independently the height of the axles  18 ,  19 , and  21  on the first side of the trailer  20  (e.g., by increasing or decreasing air in the first trailer air springs  206   a - c ), a second trailer leveling valve  212  that adjusts independently the height of the axles  18 ,  19 , and  21  on the second side of the trailer  20  (e.g., by increasing or decreasing air in the second trailer air springs  210   a - c ), a trailer cross-flow passage  216 , and a trailer cross-flow air pressure sensor  218 . In some aspects, the pneumatic circuits (e.g., first and second trailer pneumatic circuits  202  and  204 ) of the trailer system  200  illustrated in  FIG.  2 R  may be similar to the pneumatic circuits illustrated in  FIG.  2 Q . 
     In some aspects, as shown in  FIG.  2 R , the trailer system  200  may include one or more pressure sensors (e.g., pressure sensors  218 ,  220 , and  222 ). In some aspects, the one or more pressure sensors may include a first side input connected pneumatically to the first trailer air springs  206   a - 206   c  on the first side of the trailer  20  (e.g., via air line  274 ), a second side input connected pneumatically to the second trailer air springs  210   a - 210   c  on the second side of the trailer  20  (e.g., via air line  276 ), and/or a cross-flow input connected pneumatically to the trailer cross-flow passage  216 . In some aspects, the one or more pressure sensors may output first trailer air spring pressure information indicative of an air pressure within the first trailer air springs  206   a - 206   c , second trailer air spring pressure information indicative of an air pressure within the second trailer air springs  210   a - 210   c , and/or trailer cross-flow pressure information indicative of an air pressure within the trailer cross-flow passage  216 . In some aspects, the one or more pressure sensors may include one or more ADCs and/or one or more amplifiers. In some aspects, the trailer pressure information may be an analog and/or digital electrical signals. In some aspects, the computer  124  may receive the trailer pressure information and calculate a first trailer air spring-based weight indicative of a weight on the first side of the trailer  20 , a second trailer air spring-based weight indicative of a weight on the second side of the trailer  20 , and/or a trailer cross-flow-based weight. In some aspects, the pressure and/or weight information (e.g., including weight information on each of the first and second sides of the trailer  20 ) may be used (e.g., by an electronic braking system (EBS), for anti-roll control (ARC), and/or for stability control) to control brake activation based on the actual weight on each side airbag group and/or the cross-flow-based weight. In some aspects, the pressure and/or weight information (e.g., digital pressure and/or weight information) may be transmitted to any remote system (e.g., via a communication unit  139 ) through wireless (e.g., Bluetooth or wifi) or wired communication. 
     In some aspects, the trailer system  200  may include first trailer brakes  234   a - 234   c  on the first side of the trailer  20  and second trailer brakes  236   a - 236   c  on the second side of the trailer  20 . In some aspects, one or more of the trailer brakes  234   a - 234   c  and  236   a - 236   c  may include a brake booster. In some aspects, the first and second trailer brakes  234   a - 234   c  and  236   a - 236   c  may be configured to be controlled in accordance with trailer brake application levels calculated by the computer  124  of the vehicle system  100 . In some aspects, a trailer brake application level applied to one of the first trailer brakes  234   a - 234   c  may be different than one or more of the trailer brake application levels applied to the other ones of the first trailer brakes  234   a - 234   c . Similarly, in some aspects, a trailer brake application level applied to one of the second trailer brakes  236   a - 236   c  may be different than one or more of the trailer brake application levels applied to the other ones of the second trailer brakes  236   a - 236   c . In some aspects, the brake application levels applied to the first trailer brakes  234   a - 234   c  may be different than or the same as the brake application levels applied to the second trailer brakes  236   a - 236   c , respectively. 
     In some aspects, the computer  124  may apply the first and second trailer brake application levels to the first and second trailer brakes  234   a - 234   c  and  236   a - 236   c  via a valve block  260 . In some aspects, the valve block  260  may be part of the computer  124 . In some aspects, the valve block  260  may include one or more pressure modulator valves and/or one or more active braking valves (e.g., solenoid valves used for active braking) of the trailer  20 . In some aspects, the valve block  260  may include, for example, three or more (e.g., four) brake spool solenoids on each side and may, therefore, have three or more outlets on each side. In some aspects, the outlets of the valve block  260  may be connected to the first and second trailer brakes  234   a - 234   c  and  236   a - 236   c  via air lines  268   a - 268   c  and  270   a - 270   c , respectively. In some aspects, the valve block  260  may receive a constant supply of air from the one or more trailer air supply tanks  214  (e.g., via the an air line  262 ). In some aspects, the valve block  260  may include a service brake input configured to receive a brake pedal pressure signal indicative of a pressure (e.g., mechanical pressure) applied to a brake pedal of the vehicle  10  (e.g., by the foot of a driver of the vehicle  10 ). In some aspects, the valve block  260  may receive the brake pedal pressure signal via the trailer interface  134 . In some aspects, the brake pedal pressure signal may be received from a towing vehicles brake supply system of the trailer  20 . In some aspects, the brake pedal pressure signal may be a simple electrical signal or a variable millivolt (e.g., generated by a potentiometer or similar device). In some aspects, the first and second trailer brake application levels applied to the first and second trailer brakes  234   a - 234   c  and  236   a - 236   c  may be in accordance with the brake pedal pressure signal. 
     In some aspects, the computer  124  may control the valve block  260  to individually activate solenoids of the valve block  260  (e.g., to reduce brake output to prevent wheel lockup). In some aspects, the computer  124  may use speed information (e.g., output from the one or more speed sensors  232 ) and/or acceleration information (e.g., output from the one or more acceleration sensors  230 ) to individually activate solenoids of the valve block  260  for braking control. In some aspects, the computer  124  may additionally or alternatively use pressure information and/or weight information to individually activate solenoids of the valve block  260  for braking control. In some aspects, stability control, traction control, anti-lock brake system (ABS), automatic load-dependent braking (ALB), and/or anti-roll control may additionally or alternatively activate solenoids of the valve block  260  to individually supply air from the constant air supply to the first and second trailer brakes  234   a - 234   c  and  236   a - 236   c  to automatically control braking of the trailer  20 . 
     In some aspects, the one or more pressure sensors may additionally or alternatively receive pressure inputs from the first and second trailer brakes  234   a - 234   c  and  236   a - 236   c , and the one or more pressure sensors may output pressure information indicative of the air pressures within the first and second trailer brakes  234   a - 234   c  and  236   a - 236   c . In some aspects, the first and second trailer brakes  234   a - 234   c  and  236   a - 236   c  and/or scales may have analog and/or digital capability for signal pressure. In some aspects, test points may be fitted to the valve block  260  for pressure testing and fault diagnosis in each of the circuits. In some aspects, one or more display  137 , the one or more telematics units  141 , and/or the one or more user interfaces, which may be part of an ECU of an OBM system, may have system diagnostics for fault diagnosis, which may be read remotely. In some aspects, the data may have remote download capability along with GPS tracking (e.g., within the EBS electronics). In some aspects, a weigh scales interface (e.g., the one or more displays  137 ) may be configured to switch between cross-flow pressure-based information to average of first and second side pressures-based information to maintain a constant total weight. In some aspects, the vehicle system  100  and/or trailer system  200  may vent residual pressure from the cross-flow passage  116  and/or trailer cross-flow passage  216  when the leveling valves move out of cross-flow to allow for an updated pressure read, as pressures for weight can change constantly. In some aspects, if weight shifts and the cross-flow is closed, a digital read out (e.g., on the one or more displays  137 ) may split weight per side to show the operator where the load has shifted and how much weight is on each side of the vehicle  10  or the trailer  20 . 
     In some aspects, as illustrated in  FIGS.  2 S- 2 U , the trailer  20  may be a two axle trailer (e.g., a two axle dolly). In some aspects, as shown in  FIGS.  2 T and  2 U , the trailer  20  may include axles  18  and  19 . In some aspects, as shown in  FIG.  2 U , the trailer  20  may include a first trailer air spring  206   a  that supports the axle  18  on a first side, a first trailer air spring  206   b  that supports the axle  19  on the first side, a second trailer air spring  210   a  that supports the axle  18  on a second side, and a second trailer air spring  210   b  that supports the axle  19  on the second side. 
     In some aspects, as shown in  FIG.  2 U , the trailer  20  may include a first trailer leveling valve  208  that adjusts independently the height of the axle  19  on the first side of the trailer  20  (e.g., by increasing or decreasing air in the first trailer air spring  206   b ), a second trailer leveling valve  212  that adjusts independently the height of the axle  19  on the second side of the trailer  20  (e.g., by increasing or decreasing air in the second trailer air spring  210   b ), and a first trailer cross-flow passage  216   a  that connects the first trailer leveling valve  208  with the second trailer leveling valve  212 . In some aspects, the trailer  20  may include a third trailer leveling valve  280  that adjusts the height of the axle  18  on the first and second sides (e.g., by increasing or decreasing air in the first and second trailer air springs  206   a  and  210   a ), and a second trailer cross-flow passage  216   b  that connects the third trailer leveling valve  280  with the first trailer cross-flow passage  216   a  (and therefore the first and second trailer leveling valves  208  and  212 ). In some aspects, the trailer  20  may include a fitting  284  (e.g., a T-fitting) that connects the second trailer cross-flow passage  216   b  to the first trailer cross-flow passage  216   a.    
     In some aspects, as shown in  FIG.  2 U , two or more of the first, second, and third leveling trailer leveling valves  208 ,  212 , and  280  may establish pneumatic communication via the trailer cross-flow passages  216   a  and/or  216   b  when none of the two or more of the first, second, and third leveling trailer leveling valves  208 ,  212 , and  280  is adjusting the height of an axle of the trailer  20 . In some aspects, the pneumatic communication between the first, second, and third leveling trailer leveling valves  208 ,  212 , and  280  via the trailer cross-flow passages  216   a  and  216   b  may equalize air pressure between the first trailer air springs  206   a  and  206   b  and the second trailer air springs  110   a  and  110   b . In some aspects, the adjustment of the height of the axle  18  by the third leveling valve  280 , which is independent of the height adjustment of the first and second sides of the axle  19  by the first and second trailer leveling valves  208  and  212 , may reduce or prevent front to back pitching (e.g., a see saw effect) that might otherwise occur (e.g., when the trailer  20  is braking or accelerating). 
     In some aspects, as shown in  FIG.  2 U , the trailer  20  may include one or more trailer air supply tanks  214  that supply the first leveling valve  208  (e.g., via a PPV  219  and an air line  286 ), the second leveling valve  212  (e.g., via the PPV  219  and an air line  288 ), and the third leveling valve  280  (e.g., via a PPV  282  and an air line  298 ). In some aspects, the first leveling valve  208  may supply air to or remove air from the first trailer air spring  206   b  via an air line  290 , and the second leveling valve  212  may supply air to or remove air from the second trailer air spring  210   b  via an air line  292 . In some aspects, the third leveling valve  280  may supply air to or remove air from the first trailer air spring  206   a  via an air line  294 , and the third leveling valve  280  may supply air to or remove air from the second trailer air spring  210   a  via an air line  296 . 
     In some aspects, as illustrated in  FIGS.  2 V- 2 X , the trailer  20  may be a three axle trailer (e.g., a tri axle dolly). In some aspects, as shown in  FIGS.  2 W and  2 X , the trailer  20  may include axles  18 ,  19 , and  21 . In some aspects, as shown in  FIG.  2 X , the trailer  20  may include a first trailer air spring  206   a  that supports the axle  18  on a first side, a first trailer air spring  206   b  that supports the axle  19  on the first side, a first trailer air spring  206   c  that supports the axle  21  on the first side, a second trailer air spring  210   a  that supports the axle  18  on a second side, a second trailer air spring  210   b  that supports the axle  19  on the second side, and a second trailer air spring  210   c  that supports the axle  21  on the second side. 
     In some aspects, as shown in  FIG.  2 X , the trailer  20  may include a first trailer leveling valve  208  that adjusts independently the height of the axles  19  and  21  on the first side of the trailer  20  (e.g., by increasing or decreasing air in the first trailer air springs  206   b  and  206   c ), a second trailer leveling valve  212  that adjusts independently the height of the axles  19  and  21  on the second side of the trailer  20  (e.g., by increasing or decreasing air in the second trailer air springs  210   b  and  210   c ), and a first trailer cross-flow passage  216   a  that connects the first trailer leveling valve  208  with the second trailer leveling valve  212 . In some aspects, the trailer  20  may include a third trailer leveling valve  280  that adjusts the height of the axle  18  on the first and second sides (e.g., by increasing or decreasing air in the first and second trailer air springs  206   a  and  210   a ), and a second trailer cross-flow passage  216   b  that connects the third trailer leveling valve  280  with the first trailer cross-flow passage  216   a  (and therefore the first and second trailer leveling valves  208  and  212 ). In some aspects, the trailer  20  may include a fitting  284  (e.g., a T-fitting) that connects the second trailer cross-flow passage  216   b  to the first trailer cross-flow passage  216   a.    
     In some aspects, as shown in  FIG.  2 X , two or more of the first, second, and third leveling trailer leveling valves  208 ,  212 , and  280  may establish pneumatic communication via the trailer cross-flow passages  216   a  and/or  216   b  when none of the two or more of the first, second, and third leveling trailer leveling valves  208 ,  212 , and  280  is adjusting the height of an axle (or axles) of the trailer  20 . In some aspects, the pneumatic communication between the first, second, and third leveling trailer leveling valves  208 ,  212 , and  280  via the trailer cross-flow passages  216   a  and  216   b  may equalize air pressure between the first trailer air springs  206   a - 206   c  and the second trailer air springs  110   a - 110   c . In some aspects, the adjustment of the height of the axle  18  by the third leveling valve  280 , which is independent of the height adjustment of the first and second sides of the axles  19  and  21  by the first and second trailer leveling valves  208  and  212 , may reduce or prevent front to back pitching (e.g., a see saw effect) that might otherwise occur (e.g., when the trailer  20  is braking or accelerating). 
     In some aspects, as shown in  FIG.  2 X , the trailer  20  may include one or more trailer air supply tanks  214  that supply the first leveling valve  208  (e.g., via a PPV  219  and an air line  286 ), the second leveling valve  212  (e.g., via the PPV  219  and an air line  288 ), and the third leveling valve  280  (e.g., via a PPV  282  and an air line  298 ). In some aspects, the first leveling valve  208  may supply air to or remove air from the first trailer air spring  206   b  via an air line  290 , the first leveling valve  208  may supply air to or remove air from the first trailer air spring  206   c  via an air line  291 , the second leveling valve  212  may supply air to or remove air from the second trailer air spring  210   b  via an air line  292 , and the second leveling valve  212  may supply air to or remove air from the second trailer air spring  210   c  via an air line  293 . In some aspects, the third leveling valve  280  may supply air to or remove air from the first trailer air spring  206   a  via an air line  294 , and the third leveling valve  280  may supply air to or remove air from the second trailer air spring  210   a  via an air line  296 . 
       FIG.  4    is a block diagram of a non-limiting aspect of the computer  124  of the vehicle system  100 . As shown in  FIG.  4   , in some aspects, the computer  124  may include one or more processors  522  (e.g., a general purpose microprocessor) and/or one or more circuits, such as an application specific integrated circuit (ASIC), field-programmable gate arrays (FPGAs), a logic circuit, and the like. In some aspects, the computer  124  may include a data storage system (DSS)  523 . The DSS  523  may include one or more non-volatile storage devices and/or one or more volatile storage devices (e.g., random access memory (RANI)). In aspects where the computer  124  includes a processor  522 , the DSS  523  may include a computer program product (CPP)  524 . CPP  524  may include or be a computer readable medium (CRM)  526 . The CRM  526  may store a computer program (CP)  528  comprising computer readable instructions (CRI)  530 . The CRM  526  may be a non-transitory computer readable medium, such as, but not limited, to magnetic media (e.g., a hard disk), optical media (e.g., a DVD), solid state devices (e.g., random access memory (RAM) or flash memory), and the like. In some aspects, the CRI  530  of computer program  528  may be configured such that when executed by processor  522 , the CRI  530  causes the computer  124  to perform one or more of the steps described below with reference to the vehicle  10  and/or trailer  20 . In other aspects, the computer  124  may be configured to perform steps described herein without the need for a computer program. That is, for example, the computer may consist merely of one or more ASICs. Hence, the features of the aspects described herein may be implemented in hardware and/or software. 
     In some aspects, the computer  124  may be located entirely on the vehicle  10 . However, this is not required, and, in some alternative aspects, one portion of the computer  124  may be located on the vehicle  10 , and another portion of the computer  124  (e.g., one or more processors, one or more ADCs  126 , and/or one or more amplifiers  127 ) may be located on the trailer  20 . In some aspects, one or more of the pressure sensors  118 ,  120 ,  122 ,  218 ,  220 , and  222  may be pressure transducers. In some aspects, as shown in  FIGS.  2 A,  2 D,  3 A,  3 B, and  3 D , the pressure sensors  118 ,  120 ,  122 ,  218 ,  220 , and/or  222  may be separate from the computer  124 . In these aspects, the computer  124  may receive pressure information from one or more pressure sensors (e.g., either directly from the pressure sensor as an analog electrical signal or indirectly from the pressure sensor as a digital electrical signal after being converted by an ADC). However, this is not required, and, in some alternative aspects, as shown in  FIGS.  3 C and  3 E , the computer  124  may include one or more of the pressure sensors  118 ,  120 ,  122 ,  218 ,  220 , and  222 . In these aspects, computer  124  may receive one or more air flow lines as inputs. 
       FIG.  5 A  is a flow chart illustrating a braking control process  500  according to some non-limiting aspects of the invention. In some aspects, the vehicle  10  and/or trailer  20  (e.g., the computer  124  of the vehicle  10  and/or trailer  20 ) may perform one or more steps of the process  500 . 
     In some aspects, as shown in  FIG.  5 A , the process  500  may include an optional step  502  in which the first leveling valve  108  of the first pneumatic circuit  102  adjusts independently the height of the first side of the vehicle  10 . In some aspects, the first leveling valve  108  may adjust independently the height of the first side of the vehicle  10  by increasing or decreasing air in the one or more first air springs  106 . In some aspects, the first leveling valve  108  of the first pneumatic circuit  102  may adjust independently the height of the first side of the vehicle  10  to keep the vehicle  10  in (or return the vehicle  10  to) a level state. 
     In some aspects, as shown in  FIG.  5 A , the process  500  may include an optional step  504  in which the second leveling valve  112  of the second pneumatic circuit  104  adjusts independently the height of the second side of the vehicle  10 . In some aspects, the second leveling valve  112  may adjust independently the height of the second side of the vehicle  10  by increasing or decreasing air in the one or more second air springs  110 . In some aspects, the second leveling valve  112  of the second pneumatic circuit  104  may adjust independently the height of the second side of the vehicle  10  to keep the vehicle  10  in (or return the vehicle  10  to) a level state. 
     In some aspects, as shown in  FIG.  5 A , the process  500  may include an optional step  506  in which the first and second leveling valves  108  and  112  establish pneumatic communication between the first and second pneumatic circuits  102  and  104  via the cross-flow passage  116  when neither the first leveling valve  108  is adjusting independently the height of the first side of the vehicle  10  nor the second leveling valve  112  is adjusting independently the height of the second side of the vehicle  10 . 
     In some aspects, as shown in  FIG.  5 A , the process  500  may include a step  508  in which a cross-flow air pressure sensor  118  outputs cross-flow pressure information indicative of an air pressure within the cross-flow passage  116  connecting the first leveling valve  108  of the first pneumatic circuit  102  with the second leveling valve  112  of the second pneumatic circuit  104 . In some aspects, the computer  124  may receive the cross-flow pressure information. 
     In some aspects, as shown in  FIG.  5 A , the process  500  may include an optional step  510  in which the first air spring air pressure sensor  120  outputs first air spring pressure information indicative of an air pressure within the one or more first air springs  106  of the first pneumatic circuit  102 . In some aspects, as shown in  FIG.  5 A , the process  500  may include an optional step  512  in which the second air spring air pressure sensor  120  outputs second air spring pressure information indicative of an air pressure within the one or more second air springs  110  of the second pneumatic circuit  104 . In some aspects, the computer  124  may receive the first air spring pressure information and/or the second air spring pressure information. 
     In some aspects, as shown in  FIG.  5 A , the process  500  may include a step  514  in which the one or more speed sensors  132  output speed information indicative of a speed of the vehicle  10  and/or the one or more acceleration sensors  130  output acceleration information indicative of an acceleration of the vehicle  10 . In some aspects, the computer  124  may receive the speed information and/or the acceleration information. 
     In some aspects, as shown in  FIG.  5 A , the process  500  may include an optional step  516  in which the brake pedal sensor  128  outputs a brake pedal pressure signal indicative of a pressure applied to a brake pedal of the vehicle  10 . In some aspects, the computer  124  may receive the brake pedal pressure signal. 
     In some aspects, as shown in  FIG.  5 A , the process  500  may include a step  518  in which the computer  124  calculates first and second brake application levels. In some aspects, the computer  124  may calculate the first and second brake application levels using one or more of the speed and/or acceleration information and the cross-flow pressure information. In some aspects, the computer  124  may additionally use one or more of the brake pedal pressure signal, steering angle information, the first air spring pressure information, and the second air spring pressure information to calculate the first and second brake application levels. In some aspects, the computer  124  may use the speed information and the acceleration information to calculate the first and second brake application levels. In some aspects, the first and second brake application levels may be brake application pressure levels. 
     In some aspects, as shown in  FIG.  5 A , the process  500  may include a step  520  in which the computer  124  applies the first and second brake application levels. In some aspects, the computer  124  may apply the calculated first brake application level to the one or more first brakes  134  on the first side of the vehicle  10 . In some aspects, the computer  124  may apply the calculated second brake application level to the one or more second brakes  136  on the second side of the vehicle  10 . 
     In some aspects, the optional step  502  may additionally or alternatively include the first trailer leveling valve  208  of the first trailer pneumatic circuit  202  adjusting independently the height of the first side of the trailer  20 . In some aspects, the first trailer leveling valve  208  may adjust independently the height of the first side of the trailer  20  by increasing or decreasing air in the one or more first trailer air springs  206 . In some aspects, the first trailer leveling valve  208  of the first trailer pneumatic circuit  202  may adjust independently the height of the first side of the trailer  20  to keep the trailer  20  in (or return the trailer  20  to) a level state. 
     In some aspects, the optional step  504  may additionally or alternatively include the second trailer leveling valve  212  of the second trailer pneumatic circuit  204  adjusting independently the height of the second side of the trailer  20 . In some aspects, the second trailer leveling valve  212  may adjust independently the height of the second side of the trailer  20  by increasing or decreasing air in the one or more trailer second air springs  210 . In some aspects, the second trailer leveling valve  212  of the second trailer pneumatic circuit  204  may adjust independently the height of the second side of the trailer  20  to keep the trailer  20  in (or return the trailer  20  to) a level state. 
     In some aspects, the optional step  506  may additionally or alternatively include the first and second trailer leveling valves  208  and  212  establishing pneumatic communication between the first and second trailer pneumatic circuits  202  and  204  via the trailer cross-flow passage  216  when neither the first trailer leveling valve  208  is adjusting independently the height of the first side of the trailer  20  nor the second trailer leveling valve  212  is adjusting independently the height of the second side of the trailer  20 . 
     In some aspects, the step  508  may additionally or alternatively include the trailer cross-flow air pressure sensor  218  outputting trailer cross-flow pressure information indicative of an air pressure within the trailer cross-flow passage  216  connecting the first trailer leveling valve  208  of the first trailer pneumatic circuit  202  with the second trailer leveling valve  212  of the second trailer pneumatic circuit  204 . In some aspects, the computer  124  may receive the trailer cross-flow pressure information (e.g., via the trailer interface  143 ). 
     In some aspects, the step  508  may additionally or alternatively include one or more axle load sensors  111  outputting axle strain information indicative of the strain in one or more vehicle axles. In some aspects, the step  508  may additionally or alternatively include one or more trailer axle load sensors  211  outputting trailer axle strain information indicative of the strain in one or more trailer axles. 
     In some aspects, the optional step  510  may additionally or alternatively include the first trailer air spring air pressure sensor  220  outputting first trailer air spring pressure information indicative of an air pressure within the one or more first trailer air springs  206  of the first trailer pneumatic circuit  202 . In some aspects, the optional step  512  may additionally or alternatively include the second trailer air spring air pressure sensor  220  outputting second trailer air spring pressure information indicative of an air pressure within the one or more second trailer air springs  210  of the second trailer pneumatic circuit  204 . In some aspects, the computer  124  may receive the first trailer air spring pressure information and/or the second trailer air spring pressure information (e.g., via the trailer interface  143 ). 
     In some aspects, the step  514  may additionally or alternatively include the one or more trailer speed sensors  232  outputting trailer speed information indicative of a speed of the trailer  20  and/or the one or more trailer acceleration sensors  230  outputting trailer acceleration information indicative of an acceleration of the trailer  20 . In some aspects, the computer  124  may receive the trailer speed information and/or the trailer acceleration information (e.g., via the trailer interface  143 ). 
     In some aspects, in the step  518 , the computer  124  may additionally use one or more of the trailer speed information, the trailer acceleration information, the trailer cross-flow pressure information, the first trailer air spring pressure information, and the second trailer air spring pressure information to calculate the first and second brake application levels. For example, if the trailer cross flow pressure information indicates that the trailer  20  is empty (unladen), half loaded (half laden) or loaded (full laden), the computer  124  may decrease or increase the first and second brake application levels. 
     In some aspects, in the step  518 , the computer  124  may additionally or alternatively calculate first and second trailer brake application levels. In some aspects, the computer  124  may calculate the first and second trailer brake application levels using one or more of the vehicle speed information, the trailer speed information, the vehicle acceleration information, the steering angle information, the trailer acceleration information, the cross-flow pressure information, and the trailer cross-flow pressure information. In some aspects, the computer  124  may additionally use the brake pedal pressure signal, the first trailer air spring pressure information, and/or the second trailer air spring pressure information to calculate the first and second trailer brake application levels. In some aspects, the first and second trailer brake application levels may be brake application pressure levels. 
     In some aspects, using the brake pedal pressure signal, the computer  124  may calculate first and second brake application levels and/or first and second trailer brake application levels appropriate for the amount of pressure being applied to the brake pedal of the vehicle  10 . In some aspects, the calculated first and second brake application levels may be in proportion to the brake pedal pressure signal. In some aspects, the calculated first and second brake application levels and/or first and second trailer brake application levels may increase as the brake pedal pressure signal increases. In some aspects, using the speed information, the computer  124  may calculate increased first and second brake application levels appropriate for the speed at which the vehicle  10  and trailer  20  are traveling. In some aspects, the calculated first and second brake application levels may be in proportion to the speed at which the vehicle  10  and trailer  20  are traveling. In some aspects, the calculated first and second brake application may increase as the speed at which the vehicle  10  and trailer  20  are traveling increases. 
     In some aspects, the computer  124  may calculate the first and second brake application levels and/or the first and second trailer brake application levels to optimally distribute brake forces throughout the system. In some aspects, the computer  124  may calculate the first and second brake application levels and/or the first and second trailer brake application levels to perform one or more of functions of an anti-lock brake system (ABS), automatic load-dependent braking (ALB), and an electronic braking system (EBS). In some aspects, the first and second leveling valves  108  and  112  (and/or the first and second trailer leveling valves  208  and  212 ), which adjust independently the height of the first and second sides of the vehicle  10  and/or the trailer  20  to keep the vehicle  10  and/or the trailer  20  level and increase stability, may enhance one or more of the functions of the ABS, ALB, and EBS by reducing the amount and/or severity of conditions addressed by the ABS, ALB, and/or EBS. That is, the first and second leveling valves  108  and  112  (and/or the first and second trailer leveling valves  208  and  212 ) may reduce the amount and/or frequency of occurrences in which the functions of the ABS, ALB, and/or EBS are called into action. 
     In some aspects, an ABS may be designed to react to extreme braking events (e.g., by providing and maintaining the best possible traction and steering control during an extreme braking event). In some aspects, during a potential wheel lock event, the computer  124  may apply one or more brake application levels to one or more of the first and second brakes  134  and  136  and/or one or more of the first and second trailer brakes  234  and  236  to hold, apply, or release the one or more of the first and second brakes  134  and  136  and/or one or more of the first and second trailer brakes  234  and  236  as needed. In some aspects, the computer  124  may use the speed information and/or the acceleration information received from one or more speed sensors  132  and/or the one or more acceleration sensors  130  to calculate the one or more brake application levels. 
     In some aspects, the EBS may detect and control what is going on with each individual wheel set. In some aspects, the EBS may be predictive in addition to reactive (e.g., the EBS may use sensor information to predict that a dangerous situation such as jackknifing is about to occur and take action to prevent the dangerous situation from occurring). In some aspects, the EBS may include, for example and without limitation, one or more of stability control (e.g., electronic stability control (ESC), electronic stability program (ESP), dynamic stability control (DSC), and/or vehicle stability control (VSC)), traction control (e.g., automatic traction control (ATC)), ABS, ALB, and/or anti-roll control (e.g., roll stability system (RSS), roll stability program (RSP), and/or roll stability control (RSC)). In some aspects, the stability control may include measuring the yaw rate and/or lateral acceleration of the vehicle  10  and/or the trailer  20 . In some aspects, the traction control may help improve traction in low traction road conditions and/or may minimize skids (e.g., by applying different brake application levels to different wheels). In some aspects, the traction control may reduce the potential of jackknifing caused by excessive wheel spin during acceleration or in curves. In some aspects, the traction control may, when one drive wheel is spinning at a different speed than the other, momentarily apply one or more of the brakes  134 ,  136 ,  234 , and  236  until traction is regained and/or may, when both drive wheels are spinning on a poor-traction surface, automatically reduce engine power to attain optimum tire-to-road traction. 
     In some aspects, to provide automatic load-dependent braking (ALB), the computer  124  may calculate increased or decreased first and second brake application levels and/or the first and second trailer brake application levels based on whether (and/or the extent to which) the pressure information (e.g., cross-flow pressure information trailer cross-flow pressure information) indicates heavy or light loads on an axle or axle group. For instance, the computer  124  may calculate increased braking applications when the pressure information indicates a heavy load on the vehicle  10  and/or trailer  20  and decreased braking application levels when the pressure information indicates a light load (or no load) on the vehicle  10  and/or trailer  20 . 
     In some aspects, to provide anti-roll control (e.g., RSS, RSP, and/or RSC), the acceleration information output by the one or more acceleration sensors  130  and/or  230  indicates lateral acceleration of the vehicle  10  and/or trailer  20  above a roll threshold, the computer  124  may calculate the first and second brake application levels and/or the first and second trailer brake application levels to immediately brake the vehicle  10  and/or the trailer  20  and reduce the risk of skidding and/or the vehicle  10  and/or the trailer  20  tipping over. In some aspects, the anti-roll control may apply the brakes differently to particular wheels to prevent a rollover from occurring. In some aspects, when conditions indicate the potential for a rollover, the anti-roll control may reduce engine torque, engage an engine retarder, apply pressure to the brakes  134  and  136  (e.g., drive axle brakes alone or in combination with steer axle brakes), and/or modulate the trailer brakes  234  and  236  to slow the vehicle  10  and/or trailer  20  down. 
     In some aspects, the step  520  may additionally or alternatively include the computer  124  applying the first and second trailer brake application levels. In some aspects, the computer  124  may apply the calculated first trailer brake application level to the one or more first trailer brakes  234  on the first side of the trailer  20  (e.g., via the trailer interface  143 ). In some aspects, the computer  124  may apply the calculated second trailer brake application level to the one or more second trailer brakes  236  on the second side of the trailer  10  (e.g., via the trailer interface  143 ). 
     In some aspects in which the vehicle  10  includes multiple cross-flow sets and/or multiple trailer cross-flow sets, each of the steps  502 ,  504 ,  506 ,  508 ,  510 , and  512  may be performed for each of the multiple cross-flow sets and/or each of the multiple trailer cross-flow sets. In some aspects, the step  518  may include using information (e.g., cross-flow pressure information, first air spring pressure information, second air spring pressure information, trailer cross-flow pressure information, first trailer air spring pressure information, and/or second trailer air spring pressure information) from multiple cross-flow sets and/or multiple trailer cross-flow sets when calculating one or more of the vehicle and/or trailer brake application levels. 
     For example, in some aspects, the calculation of brake application levels in step  518  may include determining and using a difference (and/or a change in the difference) between (i) cross-flow pressure information indicative of an air pressure in a cross-flow passage  116  or  216  of a first cross-flow set associated with an axle or axle group that is relatively close to the front of the vehicle  10  or trailer  20  and (ii) cross-flow pressure information indicative of an air pressure in a cross-flow passage  116  or  216  of a second cross-flow set associated with an axle or axle group that is relatively close to the back of the vehicle  10  or trailer  20 . For example, in some aspects, the calculation of brake application levels in step  518  may include determining and using a difference (and/or a change in the difference) between (i) a load on a first axle or axle set of the vehicle  10  or trailer  20  that is relatively close to the front of the vehicle  10  or trailer  20  and (ii) a load on a second axle or axle set of the vehicle  10  or trailer  20  that is relatively close to the back of the vehicle  10  or trailer  20 . In some aspects, the load on the first axle or axle set may be indicated by, for example and without limitation, cross-flow pressure information or axle strain information, and the load on the second axle or axle set may be indicated by, for example and without limitation, cross-flow pressure information. 
     In some aspects, use of the cross-flow pressure information, which is indicative of the weight or mass on one or more axles of the vehicle  10  or the trailer  20  may enable vehicle and/or trailer operators to obtain on board mass information sufficient to apply for over dimension under performance based standards (PBS) and/or permits for various additional road access under a main roads framework. For example, by using cross-flow pressure information, a vehicle that is permitted to carry 9 tons on a single axle drive may be permitted to carry 10 tons after meeting the PBS as shown by the on board mass information. 
     In some aspects, the process  500  may include (e.g., in step  518 ) the computer  124  conveying digital pressure information (e.g., cross-flow pressure information, first air spring pressure information, second air spring pressure information, trailer cross-flow pressure information, first trailer air spring pressure information, and/or second trailer air spring pressure information), one or more weights calculated by the computer  124  using the pressure information, speed information, acceleration information, and/or first and second brake application levels to the one or more telematics units  141 . In some aspects, the one or more telematics units  141  may receive the information and store and/or communicate it. 
       FIG.  5 B  is a flow chart illustrating an alternative braking control process  550  according to some non-limiting aspects of the invention. In some aspects, the vehicle  10  and/or trailer  20  (e.g., the computer  124  of the vehicle  10  and/or trailer  20 ) may perform one or more steps of the process  550 . 
     In some aspects, as shown in  FIG.  5 B , the process  550  may include a step  552  in which the first leveling valve  108  of the first pneumatic circuit  102  adjusts independently the height of the first side of the vehicle  10 . In some aspects, the first leveling valve  108  may adjust independently the height of the first side of the vehicle  10  by increasing or decreasing air in the one or more first air springs  106 . In some aspects, in step  552 , the first leveling valve  108  of the first pneumatic circuit  102  may adjust independently the height of the first side of the vehicle  10  to keep the vehicle  10  in (or return the vehicle  10  to) a level state. 
     In some aspects, as shown in  FIG.  5 B , the process  550  may include a step  554  in which the second leveling valve  112  of the second pneumatic circuit  104  adjusts independently the height of the second side of the vehicle  10 . In some aspects, the second leveling valve  112  may adjust independently the height of the second side of the vehicle  10  by increasing or decreasing air in the one or more second air springs  110 . In some aspects, in step  554 , the second leveling valve  112  of the second pneumatic circuit  104  may adjust independently the height of the second side of the vehicle  10  to keep the vehicle  10  in (or return the vehicle  10  to) a level state. 
     In some aspects, as shown in  FIG.  5 B , the process  550  may include an optional step  556  in which the first and second leveling valves  108  and  112  establish pneumatic communication between the first and second pneumatic circuits  102  and  104  via the cross-flow passage  116  when neither the first leveling valve  108  is adjusting independently the height of the first side of the vehicle  10  nor the second leveling valve  112  is adjusting independently the height of the second side of the vehicle  10 . 
     In some aspects, as shown in  FIG.  5 B , the process  550  may include a step  558  of applying a brake application level to the one or more first brakes  134  on the first side of the vehicle  10  and the one or more second brakes  136  on the second side of the vehicle  10 . In some aspects, the step  558  may include applying only the same brake application level to the both first and second brakes  134  and  136 . In some aspects, the brake application level may be in proportion to a pressure applied to a brake pedal of the vehicle  10 . In some aspects, the brake application level may increase as the pressure applied to a brake pedal of the vehicle  10  (e.g., as indicated by the brake pedal pressure signal) increases. 
     In some aspects, the independent adjustment of the height of the first and second sides of the vehicle  10  in steps  552  and  554  may be so effective at keeping the vehicle  10  in (or close to) a level state that different brake application levels for the first and second brakes  134  and  136  on the first and second sides of the vehicle  10  are not necessary or are not activated under dynamic driving conditions. In some aspects, the independent adjustment of the height of the first and second sides of the vehicle  10  in steps  552  and  554  may be so effective at keeping the vehicle  10  in (or close to) a level state that one or more of ABS, ALB, and EBS are not necessary or are not activated under dynamic driving conditions. In some aspects, the independent adjustment of the height of the first and second sides of the vehicle  10  in steps  552  and  554  may be so effective at keeping the vehicle  10  in (or close to) a level state that one or more of stability control, traction control, ABS, ALB, and anti-roll control systems are not necessary or are not activated under dynamic driving conditions. 
     In some aspects, the step  558  may include a brake pedal sensor  128  outputting a brake pedal pressure signal indicative of the pressure (e.g., mechanical pressure) applied to the brake pedal of the vehicle  10  (e.g., by the foot of a driver of the vehicle  10 ). In some aspects, in step  558 , the computer  124  may receive the brake pedal pressure signal. In some aspects, the step  558  may include the computer  124  calculating the brake application level using the brake pedal pressure signal. In some aspects, the computer  124  may calculate the brake application level using one or more of the brake pedal pressure signal, speed and/or acceleration information, and cross-flow pressure information. In some alternative aspects, the computer  124  may use only the brake pedal pressure signal to calculate the brake application level. In some aspects, the computer  124  may apply the calculated brake application level to the one or more first brakes  134  on the first side of the vehicle  10  and the one or more second brakes  136  on the second side of the vehicle  10 . In some alternative aspects, a computer (e.g., the computer  124 ) may not be used in step  558  to apply the brake application level to the first and second brakes  134  and  136  (e.g., a conventional hydraulically or pneumatically controlled braking control system may be used). 
     In some aspects, the step  552  may additionally or alternatively include the first trailer leveling valve  208  of the first trailer pneumatic circuit  202  adjusting independently the height of the first side of the trailer  20 . In some aspects, the first trailer leveling valve  208  may adjust independently the height of the first side of the trailer  20  by increasing or decreasing air in the one or more first trailer air springs  206 . In some aspects, in step  552 , the first trailer leveling valve  208  of the first trailer pneumatic circuit  202  may adjust independently the height of the first side of the trailer  20  to keep the trailer  20  in (or return the trailer  20  to) a level state. 
     In some aspects, the step  554  may additionally or alternatively include the second trailer leveling valve  212  of the second trailer pneumatic circuit  204  adjusting independently the height of the second side of the trailer  20 . In some aspects, the second trailer leveling valve  212  may adjust independently the height of the second side of the trailer  20  by increasing or decreasing air in the one or more second trailer air springs  210 . In some aspects, in step  554 , the second trailer leveling valve  212  of the second trailer pneumatic circuit  204  may adjust independently the height of the second side of the trailer  20  to keep the trailer  20  in (or return the trailer  20  to) a level state. 
     In some aspects, the optional step  556  may additionally or alternatively include the first and second trailer leveling valves  208  and  212  establishing pneumatic communication between the first and second trailer pneumatic circuits  202  and  204  via the trailer cross-flow passage  216  when neither the first trailer leveling valve  208  is adjusting independently the height of the first side of the trailer  20  nor the second trailer leveling valve  212  is adjusting independently the height of the second side of the trailer  20 . 
     In some aspects, the step  558  may additionally or alternatively include applying a brake application level to the one or more first trailer brakes  234  on the first side of the trailer  20  and the one or more second trailer brakes  236  on the second side of the trailer  20 . In some aspects, the step  558  may include applying only the same brake application level to the first and second trailer brakes  234  and  236 . In some aspects, the brake application level may be in proportion to a pressure applied to a brake pedal of the vehicle  10 . In some aspects in which a brake application level is applied to the first and second brakes  134  and  136  and a brake application level is applied to the first and second trailer brakes  234  and  236 , the brake application level applied to the first and second brakes  134  and  136  may be the same as or different than the brake application level applied to the first and second trailer brakes  234  and  236 . 
     In some aspects, the process  500  may include (e.g., in step  558 ) the computer  124  conveying digital pressure information (e.g., cross-flow pressure information, first air spring pressure information, second air spring pressure information, trailer cross-flow pressure information, first trailer air spring pressure information, and/or second trailer air spring pressure information), one or more weights calculated by the computer  124  using the pressure information, speed information, acceleration information, and/or brake application level to the one or more telematics units  141 . In some aspects, the one or more telematics units  141  may receive the information and store and/or communicate it. 
     In some aspects, the independent adjustment of the height of the first and second sides of the trailer  20  in steps  552  and  554  may be so effective at keeping the trailer  20  in (or close to) a level state that different brake application levels for the first and second trailer brakes  234  and  236  on the first and second sides of the trailer  20  are not necessary. In some aspects, the independent adjustment of the height of the first and second sides of the trailer  20  in steps  552  and  554  may be so effective at keeping the trailer  20  in (or close to) a level state that one or more of ABS, ALB, and EBS are not necessary. In some aspects, the independent adjustment of the height of the first and second sides of the trailer  20  in steps  552  and  554  may be so effective at keeping the trailer  20  in (or close to) a level state that one or more of stability control, traction control, ABS, ALB, and/or anti-roll control systems are not necessary. 
     In some aspects in which the step  558  includes applying a brake application level to the first and second trailer brakes  234  and  236  of the trailer  20 , the step  558  may include a brake pedal sensor  128  outputting a brake pedal pressure signal indicative of the pressure applied to the brake pedal of the vehicle  10 . In some aspects, in step  558 , the computer  124  may receive the brake pedal pressure signal. In some aspects, the step  558  may include the computer  124  calculating the brake application level for the first and second trailer brakes  234  and  236  using the brake pedal pressure signal. In some aspects, the computer  124  may calculate the brake application level for the first and second trailer brakes  234  and  236  using one or more of the brake pedal pressure signal, speed and/or acceleration information, and cross-flow pressure information. In some alternative aspects, the computer  124  may use only the brake pedal pressure signal to calculate the brake application level for the first and second trailer brakes  234  and  236 . In some aspects, the computer  124  may apply the calculated brake application level to the one or more first trailer brakes  234  on the first side of the trailer  20  and the one or more second trailer brakes  236  on the second side of the trailer  20 . In some alternative aspects in which the step  558  includes applying a brake application level to the first and second trailer brakes  234  and  236  of the trailer  20 , a computer (e.g., the computer  124 ) may not be used in step  558  to apply the brake application level to the first and second trailer brakes  234  and  236  (e.g., a conventional hydraulically or pneumatically controlled braking control system may be used). 
       FIG.  6    is a flow chart illustrating a vehicle load monitoring process  600  according to some non-limiting aspects of the invention. In some aspects, the vehicle  10  and/or trailer  20  (e.g., the computer  124  of the vehicle  10  and/or trailer  20 ) may perform one or more steps of the process  600 . 
     In some aspects, as shown in  FIG.  6   , the process  600  may include an optional step  602 , an optional step  604 , an optional step  606 , a step  608 , an optional step  610 , and an optional step  612 , which may be the same as the optional step  502 , the optional step  504 , the optional step  506 , the step  508 , the optional step  510 , and the optional step  512 , respectively, of the process  500  described above with respect to  FIG.  5 A . 
     In some aspects, as shown in  FIG.  6   , the process  600  may include a step  614  in which an ADC  126  converts the cross-flow pressure information into digital cross-flow pressure information. In some aspects, the step  614  may include an ADC  126  converting the first air spring pressure information into digital first air spring pressure information. In some aspects, the step  614  may include an ADC  126  converting the second air spring pressure information into digital second air spring pressure information. 
     In some aspects, the step  614  may include an ADC  126  converting trailer cross-flow pressure information into digital trailer cross-flow pressure information. In some aspects, the step  614  may include an ADC  126  converting the first trailer air spring pressure information into digital first trailer air spring pressure information. In some aspects, the step  614  may include an ADC  126  converting the second trailer air spring pressure information into digital second trailer air spring pressure information. 
     In some aspects, the step  614  may include an ADC  126  converting axle strain information into digital axle strain information. In some aspects, the step  614  may include an ADC  126  converting trailer axle strain information into digital trailer axle strain information. 
     In some aspects, as shown in  FIG.  6   , the process  600  may include a step  616  in which a display (e.g., a display  137 ) displays the digital cross-flow pressure information. In some aspects, the step  616  may include the display displaying the digital first air spring pressure information and/or the digital second air spring pressure information (e.g., simultaneously with the digital cross-flow pressure information). 
     In some aspects, the step  616  may include the display displaying the digital trailer cross-flow pressure information (e.g., simultaneously with the digital cross-flow pressure information). In some aspects, the step  616  may include the display displaying the digital first trailer air spring pressure information and/or the digital second trailer air spring pressure information (e.g., simultaneously with the digital trailer cross-flow pressure information). In some aspects, the step  616  may include the display displaying the digital axle strain information and/or the digital trailer axle strain information. 
     In some aspects, the step  616  may additionally or alternatively include the one or more printers  138  printing any of the digital information (e.g., the digital cross-flow pressure information and/or the digital trailer cross-flow pressure information). In some aspects, the step  616  may additionally or alternatively include the one or more communication units  139  communicating (e.g., wirelessly communicating) any of the digital information (e.g., for display on a remote device). 
     In some aspects, the process  600  may include (e.g., in step  616 ) the computer  124  conveying digital pressure information (e.g., cross-flow pressure information, first air spring pressure information, second air spring pressure information, trailer cross-flow pressure information, first trailer air spring pressure information, and/or second trailer air spring pressure information) to the one or more telematics units  141 . In some aspects, the one or more telematics units  141  may receive the information and store and/or communicate it. 
     In some aspects in which the vehicle  10  includes multiple cross-flow sets and/or multiple trailer cross-flow sets, each of the steps  602 ,  604 ,  606 ,  608 ,  610 ,  612 ,  614 , and  616  may be performed for each of the multiple cross-flow sets and/or each of the multiple trailer cross-flow sets. In some aspects, the step  618  may include the display displaying (e.g., simultaneously) digital information for multiple cross-flow sets and/or multiple trailer cross-flow sets. For example, in step  618 , the display may display digital cross-flow pressure information for each of multiple cross-flow sets and/or digital trailer cross-flow pressure information for each of multiple trailer cross-flow sets. 
     In this way, a vehicle operator may be able to view displayed information indicative of the cross-flow pressure associated with one or more vehicle axles, one or more groups of vehicle axles, one or more trailer axles, and one or more groups of trailer axles. In some aspects, the display (e.g., a display  137 ) may display (i) digital axle strain information associated with one or more axles (e.g., axle  12 ) of the vehicle  10 , (ii) digital cross-flow pressure information associated with one or more axles of the vehicle  10  (e.g., axle  12  individually as shown in  FIG.  2 C ) and/or one or more groups of axles of the vehicle  10  (e.g., group of axles  14  and  15  as shown in  FIGS.  2 B and  2 C ), (iii) digital trailer axle strain information associated with one or more axles of the trailer  20  (e.g., axle  16 ), and/or (iv) digital trailer cross-flow pressure information associated with one or more axles of the trailer  20  (e.g., axles  18 ,  19 , and  21  individually as shown in  FIG.  2 F ) and/or one or more groups of axles of the trailer  20  (e.g., group of axles  18  and  19  as shown in  FIG.  2 E ). 
     In some aspects, as shown in  FIG.  7   , the process  700  may include an optional step  702 , an optional step  704 , an optional step  706 , a step  708 , an optional step  710 , an optional step  712 , and a step  714 , which may be the same as the optional step  502 , the optional step  504 , the optional step  506 , the step  508 , the optional step  510 , the optional step  512 , and the step  614 , respectively, described above with respect to  FIGS.  5  and  6   . 
     In some aspects, as shown in  FIG.  7   , the process  700  may include a step  714  in which the computer  124  calculates a cross-flow-based weight on one or more axles of the vehicle  10  (e.g., an axle mass or an axle group mass). In some aspects, the computer  124  may calculate the cross-flow-based weight using the digital cross-flow pressure information. In some aspects, in the step  714 , the computer  124  may additionally or alternatively calculate a trailer cross-flow-based weight on one or more axles of the trailer  20 . In some aspects, the computer  124  may calculate the trailer cross-flow-based weight using the digital trailer cross-flow pressure information. In some aspects, the computer  124  that calculates the cross-flow-based weight in step  714  may be the same computer  124  that calculates the first and second brake application levels and/or the first and second trailer brake application levels to perform one or more of functions of an anti-lock brake system (ABS), automatic load-dependent braking (ALB), and/or an electronic braking system (EBS). In some aspects, the computer  124  may calculate the cross-flow-based weight in step  714  as a part of performing one or more of functions of an ABS, ALB, and/or an EBS (e.g., stability control, traction control, ALB, and/or anti-roll control). In some aspects, the cross-flow-based weight calculated by the computer  124  may be accurate enough (e.g., within ±2%) for the vehicle  10  and/or the trailer  20  to meet the requirements of one or more performance based standards (PBS) without the vehicle  10  and/or the trailer  20  being equipped with any on board mass units and/or load cells, which are expensive and are conventionally needed to meet the performance based standards. 
     In some aspects, the step  714  may include the computer  124  calculating a first air spring-based weight using the digital first air spring pressure information and/or calculating a second air spring-based weight using the digital second air spring pressure information. In some aspects, the step  714  may include the computer  124  calculating a first trailer air spring-based weight using the digital first trailer air spring pressure information and/or calculating a second trailer air spring-based weight using the digital second trailer air spring pressure information. 
     In some aspects, the step  714  may include the computer  124  calculating a strain-based weight on one or more axles (e.g., axle  12 ) of the vehicle  10 . In some aspects, the computer  124  may calculate the strain-based weight using the digital axle strain information. In some aspects, the step  714  may additionally or alternatively include the computer  124  calculating a strain-based weight on one or more axles (e.g., axle  16 ) of the trailer  20 . In some aspects, the computer  124  may calculate the strain-based weight using the digital trailer axle strain information. 
     In some aspects, the computer  124  may receive reference weight information (e.g., one or more reference weight measurements). In some aspects, the computer  124  may include the reference weight information (e.g., via the communication unit  139  or a user input). In some aspects, the computer  124  may use the reference weight information to calibrate the one or more weight calculations of step  716 , which may calculate one or more weights at a first measurement time. 
     In some aspects, the computer  124  may receive first reference weight information indicative of a weight on one or more axles of the vehicle  10  or trailer  20  at a second measurement time (e.g., a time when the vehicle  10  or trailer  20  is unloaded). In some aspects, the first reference weight information may be obtained by moving the vehicle  10  or trailer  20  in an unloaded state onto a scale. In some aspects, the system  100  may generate measurement information at the second measurement time so that is corresponds to the first reference weight information. In some aspects, the measurement information may including one or more of cross-flow pressure information, first air spring pressure information, second air spring pressure information, axle strain information, trailer cross-flow pressure information, first trailer air spring pressure information, second trailer air spring pressure information, and trailer axle strain information at the second measurement time. In some aspects, the system  100  may use one or more ADCs  126  to convert that measurement information to digital measurement information. In some aspects, the system  100  may use the first reference weight information and the corresponding measurement information at the second measurement time to calculate the one or more weights at the first measurement time in step  716 . 
     In some aspects, the computer  124  may receive a second reference weight information indicative of a weight on one or more axles of the vehicle  10  or trailer  20  at a third measurement time (e.g., a time when the vehicle  10  or trailer  20  is loaded). In some aspects, the second reference weight information may be obtained by moving the vehicle  10  or trailer  20  in a loaded state onto the scale. In some aspects, the system  100  may generate measurement information at the third measurement time so that is corresponds to the second reference weight information. In some aspects, the system  100  may use one or more ADCs  126  to convert that measurement information to digital measurement information. In some aspects, the system  100  may use the first and second reference weight information and the corresponding measurement information at the second and third measurement times to calculate the one or more weights at the first measurement time in step  716 . 
     In some aspects, the computer  124  may use the one or more reference weight information and the corresponding measurement information to calculate one or more pressure-to-weight conversion functions (e.g., a cross-flow-pressure-to-weight conversion function) and/or one or more strain-to-weight functions. In some aspects, the computer  124  may calculate one or more weights on one or more axles at the first measurement time using the one or more conversion functions and measurement information at the first measurement time. In some aspects in which the computer  124  receives at least first and second reference weight information, the computer  124  may use linear interpolation to calculate the one or more conversion functions. In some alternative aspects in which the computer  124  receives at least first, second, and third reference weight information, the computer  124  may use polynomial interpolation to calculate the one or more conversion functions. 
     In some aspects, as shown in  FIG.  7   , the process  700  may include a step  718  in which a display (e.g., a display  137 ) displays the cross-flow-based weight. In some aspects, the step  718  may include the display displaying the first air spring-based weight and/or the second air spring-based weight (e.g., simultaneously with the cross-flow-based weight). 
     In some aspects, the step  718  may include the display displaying the trailer cross-flow-based weight (e.g., simultaneously with the cross-flow-based weight). In some aspects, the step  718  may include the display displaying the first trailer air spring-based weight and/or the second trailer air spring-based weight (e.g., simultaneously with the trailer cross-flow-based weight). In some aspects, the step  718  may include the display displaying the strain-based weight and/or the trailer strain-based weight. 
     In some aspects, the step  718  may additionally or alternatively include the one or more printers  138  printing any of the calculated weights (e.g., the cross-flow-based weight and/or the trailer cross-flow-based weight). In some aspects, the step  718  may additionally or alternatively include the one or more communication units  139  communicating (e.g., wirelessly communicating) any of the calculated weights (e.g., for display on a remote device). 
     In some aspects, the process  700  may include (e.g., in step  718 ) the computer  124  conveying digital pressure information (e.g., cross-flow pressure information, first air spring pressure information, second air spring pressure information, trailer cross-flow pressure information, first trailer air spring pressure information, and/or second trailer air spring pressure information) and/or one or more weights calculated by the computer  124  using the pressure information to the one or more telematics units  141 . In some aspects, the one or more telematics units  141  may receive the information and store and/or communicate it. 
     In some aspects in which the vehicle  10  includes multiple cross-flow sets and/or multiple trailer cross-flow sets, each of the steps  702 ,  704 ,  706 ,  708 ,  710 ,  712 ,  714 , and  716  may be performed for each of the multiple cross-flow sets and/or each of the multiple trailer cross-flow sets. In some aspects, the step  718  may include the display displaying (e.g., simultaneously) calculated weights for multiple cross-flow sets and/or multiple trailer cross-flow sets. For example, in step  718 , the display may display a cross-flow-based weight for each of multiple cross-flow sets and/or a trailer cross-flow-based weight for each of multiple trailer cross-flow sets. 
     In this way, a vehicle operator may be able to view displayed cross-flow-based weight indicative of the weights one or more vehicle axles, one or more groups of vehicle axles, one or more trailer axles, and one or more groups of trailer axles. In some aspects, the display (e.g., a display  137 ) may display (i) one or more strain-based weights on one or more axles (e.g., axle  12 ) of the vehicle  10 , (ii) one or more cross-flow-based weights on one or more axles of the vehicle  10  (e.g., axle  12  individually as shown in  FIG.  2 C ) and/or one or more groups of axles of the vehicle  10  (e.g., group of axles  14  and  15  as shown in  FIGS.  2 B and  2 C ), (iii) one or more trailer strain-based weights on one or more axles of the trailer  20  (e.g., axle  16 ), and/or (iv) one or more trailer cross-flow-based weights on one or more axles of the trailer  20  (e.g., axles  18 ,  19 , and  21  individually as shown in  FIG.  2 F ) and/or one or more groups of axles of the trailer  20  (e.g., group of axles  18  and  19  as shown in  FIG.  2 E ). 
     In some aspects, the step  716  may include the computer  124  calculating one or more of a gross vehicle weight, a gross trailer weight, and a gross combined weight. In some aspects, the gross vehicle weight may be a combined weight on all of the axles and/or axle groups of the vehicle  10 . In some aspects, the computer  124  may calculate the gross vehicle weight by summing the weights of the axles and/or axle groups of the vehicle  10 . For example, in the aspect shown in  FIG.  2 B , the computer  124  may calculate the gross vehicle weight as the sum of the strain-based weight on the axle  12  and the cross-flow-based weight on the group of axles  14  and  15 . In the aspect shown in  FIG.  2 C , the computer  124  may calculate the gross vehicle weight as the sum of the cross-flow-based weight on the axle  12  and the cross-flow-based weight on the group of axles  14  and  15 . 
     In some aspects, the gross trailer weight may be the combined weight on all of the axles and/or axle groups of the trailer  20 . In some aspects, the computer  124  may calculate the gross trailer weight by summing the weights of the axles and/or axle groups of the trailer  20 . For example, in the aspect shown in  FIG.  2 E , the computer  124  may calculate the gross trailer weight as the sum of the strain-based weight on the axle  16  and the cross-flow-based weight on the group of axles  18  and  19 . In the aspect shown in  FIG.  2 F , the computer  124  may calculate the gross trailer weight as the sum of the cross-flow-based weight on the axle  18 , the cross-flow-based weight on the axle  19 , and the cross-flow-based weight on the axle  21 . 
     In some aspects, the gross combined weight may be the combined weight on all of the axles and/or axle groups of the vehicle  10  and the trailer  20 . In some aspects, the gross combined weight may be equal to the sum of the gross vehicle weight and the gross trailer weight. 
     In some aspects, the step  716  may include the computer  124  additionally or alternatively calculating one or more of a vehicle load weight, a trailer load weight, and a combined load weight. In some aspects, the vehicle load weight may be the current gross vehicle weight minus the gross vehicle weight in the unloaded state. In some aspects, the trailer load weight may be the current gross trailer weight minus the gross trailer weight in the unloaded state. In some aspects, the combined load weight may be equal to the sum of the vehicle load weight and the trailer load weight. 
     In some aspects, the step  718  may include the display (e.g., a display  137 ) displaying one or more of the gross vehicle weight, gross trailer weight, gross combined weight, vehicle load weight, trailer load weight, and combined load weight. 
     Experimental Data 
     Testing was carried out to compare (a) cross-flow-based weights calculated (e.g., by a computer  124 ) using cross-flow pressure information (e.g., output by a cross-flow air pressure sensor  118  or a trailer cross-flow air pressure sensor  218 ) indicative of an air pressure within a cross-flow passage (e.g., cross-flow passage  116  or trailer cross-flow passage  216 ) and (b) air spring-based weights calculated (e.g., by a computer  124 ) using air spring pressure information (e.g., output by an air spring air pressure sensor  120 ,  122 ,  220 , or  222 ) indicative of an air pressure within an air spring (e.g., air spring  106 ,  110 ,  206 , or  210 ) on one side of a vehicle or trailer. The measurements were taken using a system  100  embodying aspects of the present invention with the only difference being whether the pressure sensor measured the air pressure within the cross-flow passage or within an air spring on one side of the vehicle or trailer. 
     In one test, a trailer  20  was loaded with cattle. During dynamic driving conditions, cattle can slide and move around a trailer  20 , making it difficult to keep the vehicle  10  and/or the trailer  20  stable. Measurements were taken with the trailer  20  loaded with cattle and traveling at a speed of over 20 km/hr. A trip was defined as (i) starting when an axle load sum was over 12 metric tons (T) and a speed was over 20 km/hr (i.e., when there were cattle on the trailer  20  and the trailer  20  was moving) and (ii) ending when the axle load sum drops down below 12 T. 
     As shown in Table 1 below, during three trips in which trailer air spring-based axle weights were calculated using trailer air spring pressure information indicative of an air pressure within an air spring on a side of the trailer  20 , the calculated trailer axle weights (e.g., calculated trailer axle load sums) varied by 3400 kg, 4500 kg, and 4200 kg, respectively, even though the trailer axle weights did not actually change by those amounts during the trips. In contrast, during the trip in which trailer cross-flow-based axle weights were calculated using trailer cross-flow pressure information indicative of an air pressure within a trailer cross-flow passage  216  of the trailer  20 , the calculated trailer axle weights (e.g., calculated trailer axle load sums) varied by only 800 kg. Moreover, the lowest trailer cross-flow-based axle weight was calculated at the start of the trip, and, once the trailer  20  was traveling at over 40 km/h, the calculated trailer cross-flow-based axle weights varied by only 400 kg. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Comparison of Air Spring-Based and Cross-Flow-Based Weights 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Difference 
               
               
                   
                   
                   
                   
                 between Max. 
               
               
                   
                   
                 Minimum 
                 Maximum 
                 and Min. 
               
               
                   
                 Trip 
                 Calculated 
                 Calculated 
                 Calculated 
               
               
                 Type 
                 # 
                 Weight 
                 Weight 
                 Weights 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Air Spring-Based Axle 
                 1 
                 25,000 kg 
                 28,400 kg 
                 3,400 kg 
               
               
                 Weights Calculated 
                 2 
                 21,900 kg 
                 26,400 kg 
                 4,500 kg 
               
               
                 using Air Spring 
                 3 
                 22,000 kg 
                 26,300 kg 
                 4,300 kg 
               
               
                 Pressure Information 
               
               
                 in System with 
               
               
                 Cross-Flow Passage 
               
               
                 Cross-Flow-Based Axle 
                 4 
                 26,300 kg 
                 27,100 kg 
                   800 kg 
               
               
                 Weights Calculated 
               
               
                 using Cross-Flow 
               
               
                 Pressure Information 
               
               
                 in System with 
               
               
                 Cross-Flow Passage 
               
               
                   
               
            
           
         
       
     
     In some aspects, as the cross-flow passage  116  (or  216 ) is not in pneumatic communication with the first and second pneumatic circuits  102  and  104  (or  202  and  204 ) when the first and/or second leveling valves  108  and  112  (or  208  and  212 ) are adjusting independently the height of the first and/or second sides of the vehicle  10  (or the trailer  20 ), the air pressure within the cross-flow passage  116  (or  216 ) is consistent and may not be affected by events (e.g., lateral acceleration, surface unevenness, etc.) that cause the first and/or second leveling valves  108  and  112  (or  208  and  212 ) to adjust independently the height of the first and/or second sides of the vehicle  10  (or the trailer  20 ). Accordingly, the air pressure within the cross-flow passage  116  (or  216 ) may be an accurate representation of air spring suspension pressure. 
     The measurements taken during the trip in which trailer cross-flow-based axle weights were calculated using trailer cross-flow pressure information indicative of an air pressure within a trailer cross-flow passage  216  of the trailer  20  bear this out. In particular, the calculated trailer cross-flow-based axle weights showed that the air pressure within the trailer cross-flow passage  216  was not affected by even significant shifts in lateral acceleration of the trailer  20 . In contrast, for calculated trailer air spring-based axle weights, which were calculated based on the measured air pressure within one or more trailer air springs  206  or  210  instead of based on the measured air pressure within the trailer cross-flow passage  216 , the amplitude of difference in calculated trailer axle weights was 425% to 562% higher compared to the difference in trailer axle weights calculated using the trailer cross-flow passage  216 , demonstrating the inaccuracies that exist in conventional systems that rely on air spring pressure information. Accordingly, the experimental data shows that the air pressure within the cross-flow passage  116  or  216  is a more consistent and more accurate representation of the air pressure of the complete air spring suspension system including the first and second pneumatic circuits  102  and  104  (or  202  and  204 ) than the air pressure within one or more air springs on one side of the vehicle  10  (or trailer  20 ). Thus, use of the air pressure within the cross-flow passage  116  or  216  for load monitoring and/or braking control provides advantages over the conventional use of the air pressure within one or more air springs on one side of the vehicle  10  (or trailer  20 ). 
     Raise Lower Valve (RLV) Configuration 
       FIGS.  8 A- 8 E  illustrate a system including a leveling valve  860  (e.g., the leveling valve  108 ,  112 ,  208 , or  212 ) and a raise lower valve (RLV)  800  (e.g., RLV  107  or  207 ) according to some aspects. In some aspects, the leveling valve  860  may be configured to adjust independently the height of a side of a vehicle  10  or trailer  20 . In some aspects, the leveling valve  860  may adjust the height of the one side of the vehicle  10  or trailer  20  by increasing or decreasing air in one or more air springs  870  (e.g., one or more air springs  106 ,  110 ,  206 , or  210 ) on the one side of the vehicle  10  or trailer  20 . In some aspects, the RLV  800  may enable an operator to manually vary the height of the chassis of the vehicle  10  or trailer  20  relative to the ground (e.g., to facilitate height adjustment of the vehicle or trailer at a loading dock). 
     In some aspects, as shown in  FIGS.  8 A- 8 E , the leveling valve  860  may include one or more ports  862 . In some aspects, as shown in  FIGS.  8 A- 8 E , the leveling valve  860  may include a supply port  862   a , an exhaust (or dump or vent) port  862   b , a cross-flow port  862   c , and/or one or more spring ports  862   d  and  862   e . In some aspects, the supply port  862   a  of the leveling valve  860  may be configured to receive air from an air source such as one or more air supply tanks  866  (e.g., the one or more air supply tanks  114  or  214 ). In some aspects, a pressure protection valve (PPVs)  868  may be located between the one or more air supply tanks  866  and the supply port  862   a  of the leveling value  860 , and the PPV  868  may protect the system in the event of a leak or failure within the system. However, the PPV  868  is not required, and, in some alternative aspects, the system may not include a PPV. In some aspects, the exhaust port  862   b  may be configured to exhaust air into the atmosphere. In some aspects, the one or more spring ports  862   d  and  862   e  may be configured to receive air from or supply air to the one or more air springs  870 . In some aspects, the spring port  862   e  may be plugged. 
     In some aspects, the cross-flow port  862   c  may be configured to connect the leveling valve  860  with a leveling valve on a pneumatic circuit on the other side of the vehicle or trailer via a cross-flow passage  864  (e.g., the cross-flow passage  116  or  216 ). In some aspects, the leveling valves may be configured to establish pneumatic communication via the cross-flow passage  864  when the first and second leveling valves are allowed to adjust independently the heights of the first and second sides of the vehicle or the trailer but neither the first leveling valve is adjusting independently the height of the first side nor the second leveling valve is adjusting independently the height of the second side neither leveling valve is adjusting independently the height of a side of the vehicle  10  or trailer  20 . In some aspects, the pneumatic communication between the pneumatic circuits via the cross-flow passage  864  may equalize air pressure between air springs  870  on opposite sides of the vehicle  10  or trailer  20 . 
     In some aspects, the RLV  800  may include an RLV user interface  802 , a communication interface  803 , an RLV controller  804 , a first RLV actuator  806 , a second RLV actuator  808 , a first RLV housing  814 , and/or a second RLV housing  816 . In some aspects, as shown in  FIGS.  8 A- 8 E , the first and second RLV housings  814  and  816  may be separate and distinct housings. In some alternative aspects, the first and second RLV housings  814  and  816  may be a single, integrated housing. In some aspects, the first RLV actuator  806  may be a three position solenoid including first and second solenoids  810   a  and  810   b , and the second RLV actuator  808  may be a three position solenoid including first and second solenoids  812   a  and  812   b.    
     In some aspects, the first RLV housing  814  may include a supply port  818  and/or an exhaust (or dump or vent) port  820 . In some aspects, the second RLV housing  816  may include a spring port  822 . In some aspects, the supply port  818  of the first RLV housing  814  may be configured to receive air from an air source such as the one or more air supply tanks  866  (e.g., either directly or indirectly through the PPV  868 ). In some aspects, an air line may connect the source port  818  of the first RLV housing  814  and the PPV  868  (or an air supply tank  866 ). In some aspects, the exhaust port  820  of the first RLV housing  814  may be configured to exhaust air into the atmosphere. In some aspects, the spring port  822  of the second RLV housing  816  may be configured to receive air from or supply air to the one or more air springs  870 . 
       FIGS.  8 A- 8 E  provide a cross-sectional view of the first and second RLV housings  814  and  816 . In some aspects, as shown in  FIGS.  8 A- 8 E , the first RLV housing  814  may include a main passage  824  (e.g., a central passage or bore), a supply passage  826 , a leveling valve supply passage  828 , a leveling valve bypass passage  830 , and/or an exhaust passage  832 . In some aspects, the first RLV housing  814  may include a first supply seal  842  and a second supply seal  844  in the main passage  824 . In some aspects, a connector  844  may connect the first and second supply seals  842  and  844  and may maintain the first and second supply seals  842  and  844  a constant distance apart from each other. In some aspects, the first RLV actuator  806  may be configured to move the first and second supply seals  842  and  844  between neutral, raise, and lower positions.  FIGS.  8 A and  8 C  illustrate the first and second supply seals  842  and  844  of the first RLV housing  814  in a neutral position.  FIG.  8 B  illustrates the first and second supply seals  842  and  844  in a raise position.  FIGS.  8 D and  8 E  illustrate the first and second supply seals  842  and  844  of the first RLV housing  814  in a lower position. In some aspects, one or more of the first and second supply seals  842  and  844  may include magnetic material. In some aspects in which the first RLV actuator  806  is a three position solenoid including first and second solenoids  810   a  and  810   b , an electrical current passing through one of the first and second solenoids  810   a  and  810   b  may generate a magnetic field that moves the first and second supply seals  842  and  844  from the neutral position shown in  FIG.  8 A  to the raise position shown in  FIG.  8 B , and an electrical current passing through the other one of the first and second solenoids  810   a  and  810   b  may generate a magnetic field that moves the first and second supply seals  842  and  844  from the neutral position shown in  FIG.  8 A  to the lower position shown in  FIG.  8 D . In some aspects, springs (not shown) in the main passage  824  above and/or below the first and second supply seals  842  and  844  may cause the first and second supply seals  842  and  844  to return to their neutral position as shown in  FIGS.  8 A and  8 C  when neither of the first and second solenoids  810   a  and  810   b  is generating a magnetic field. 
     In some aspects, the main passage  824  of the first RLV housing  814  may connect pneumatically with the supply port  862   a  of the leveling valve  860 . In some aspects, as shown in  FIGS.  8 A- 8 E , the first RLV housing  814  may be attached to the supply port  862   a  of the leveling value  860  so that the main passage  824  of the first RLV housing  814  connects directly with the supply port  862   a  of the leveling valve  860 . However, this is not required, and, in some alternative aspects, the first RLV housing  814  may not be attached to the supply port  862   a  of the leveling value  860 , and an air line may connect pneumatically the main passage  824  of the first RLV housing  814  with the supply port  862   a  of the leveling valve  860 . 
     In some aspects, the supply passage  826  of the first RLV housing  814  may connect pneumatically the supply port  818  and the main passage  824 . In some aspects, the leveling valve supply passage  828  may connect pneumatically the main passage  824  above the second supply seal  844  when in the neutral position to the main passage  824  below the second supply seal  844  when the second supply seal  844  is in the neutral position, as shown in  FIGS.  8 A and  8 C . In some aspects, the exhaust passage  832  may connect pneumatically the main passage  824  and the exhaust port  820  of the first RLV housing  814 . In some aspects, the leveling valve bypass passage  830  may be part of a bypass path that connects pneumatically the main passage  824  of the first RLV housing  814  and a main passage  834  of the second RLV housing  816 . 
     In some aspects, as shown in  FIGS.  8 A- 8 E , the second RLV housing  816  may include the main passage  834  (e.g., a central passage or bore), a leveling valve bypass passage  836 , a leveling valve spring passage  838 , and/or a spring passage  840 . In some aspects, the second RLV housing  816  may include a first delivery seal  846  and a second delivery seal  848  in the main passage  834 . In some aspects, a connector  847  may connect the first and second delivery seals  846  and  848  and may maintain the first and second seals delivery  846  and  848  a constant distance apart from each other. In some aspects, the second RLV actuator  808  may be configured to move the first and second delivery seals  846  and  848  between neutral, raise, and lower positions.  FIGS.  8 A and  8 E  illustrate the first and second delivery seals  846  and  848  of the second RLV housing  816  in a neutral position.  FIGS.  8 B and  8 C  illustrate the first and second delivery seals  846  and  848  in a raise position.  FIG.  8 D  illustrates the first and second delivery seals  846  and  848  of the second RLV housing  816  in a lower position. In some aspects, one or more of the first and second delivery seals  846  and  848  may include magnetic material. In some aspects in which the second RLV actuator  808  is a three position solenoid including first and second solenoids  812   a  and  812   b , an electrical current passing through one of the first and second solenoids  812   a  and  812   b  may generate a magnetic field that moves the first and second delivery seals  846  and  848  from the neutral position shown in  FIG.  8 A  to the raise position shown in  FIG.  8 B , and an electrical current passing through the other one of the first and second solenoids  812   a  and  812   b  may generate a magnetic field that moves the first and second delivery seals  846  and  848  from the neutral position shown in  FIG.  8 A  to the lower position shown in  FIG.  8 D . In some aspects, springs (not shown) in the main passage  834  above and/or below the first and second delivery seals  846  and  848  may cause the first and second delivery seals  846  and  848  to return to their neutral position as shown in  FIGS.  8 A and  8 E  when neither of the first and second solenoids  812   a  and  812   b  is generating a magnetic field. 
     In some aspects, the main passage  834  of the second RLV housing  816  may connect pneumatically with the spring port  862   d  of the leveling valve  860 . In some aspects, as shown in  FIGS.  8 A- 8 E , the second RLV housing  816  may be attached to the supply port  862   a  of the leveling value  860  so that the main passage  834  of the second RLV housing  816  connects directly with the spring port  862   d  of the leveling valve  860 . However, this is not required, and, in some alternative aspects, the second RLV housing  816  may not be attached to the spring port  862   d  of the leveling value  860 , and an air line may connect pneumatically the main passage  834  of the second RLV housing  816  with the spring port  862   d  of the leveling valve  860 . 
     In some aspects, the leveling valve bypass passage  836  of the second RLV housing  816  may be part of the bypass path that connects pneumatically the main passage  824  of the first RLV housing  814  and the main passage  834  of the second RLV housing  816 . In some aspects, the leveling valve spring passage  838  may connect pneumatically the main passage  834  above the second delivery seal  848  when in the neutral position to the main passage  834  below the second delivery seal  848  when in the neutral position, as shown in  FIGS.  8 A and  8 E . In some aspects, a spring passage  840  may connect pneumatically the main passage  834  and the spring port  822  of the second RLV housing  816 . 
     In some aspects including first and second RLV housings  814  and  816 , a bypass air line  850  may connect pneumatically the leveling valve bypass passage  836 . In these aspects, the bypass path that connects pneumatically the main passages  824  and  834  of the first and second RLV housings  814  and  816  may include the leveling valve bypass passage  830  of the first RLV housing  814 , the bypass air line  850 , and the leveling valve bypass passage  836  of the second RLV housing  816 . In some alternative aspects including a single integrated housing, the bypass path that connects pneumatically the main passages  824  and  834  of the integrated RLV housing may include a single leveling valve bypass passage. 
     In some aspects, the RLV controller  804  may receive input from the RLV user interface  802  and/or the communication interface  803 . In some embodiments, communication interface  803  may communicate with one or more devices (e.g., the computer  124 , the communication interface  139 , and/or another computer such as a smartphone running an RLV application) through wired and/or wireless communication. In some aspects, the vehicle communicates (e.g., automatically) with a location, e.g., a loading dock, using one or more sensors (e.g., proximity sensors), cameras, lasers, radar, lidar (Light Detection and Ranging), wireless communication protocols (e.g., Bluetooth) to determine the appropriate height setting and the RLV responds accordingly to raise or lower the vehicle. In some aspects, the vehicle is an autonomous vehicle such that no user/driver is required to operate the system. 
     In some aspects, the RLV user interface  802  may include a user input. In some aspects, the user input may include a switch. In some aspects, the switch may a three-position switch including a lower position (e.g., left position), neutral position (e.g., center position), and raise position (e.g., right position). In some aspects, the user input may additionally or alternatively include a raise button and a lower button. In some aspects, user input (e.g., one or more switches and/or one or more buttons) of the RLV user interface  802  may be implemented using physical switches and/or physical buttons and/or a touchscreen having a graphical user interface that may be manipulated by touch or by another control device. In some aspects, the RLV controller  804  may receive user input from the RLV user interface  802 . In some aspects, the RLV controller  804  may control one or more of the first and second RLV actuators  806  and  808  based on the input received from the RLV user interface  802  and/or the communication interface  803 . In some aspects, the RLV controller  804  may be part of the computer  124  of the vehicle  10  and/or trailer  20 . 
     In some aspects, the RLV controller  804  may include a timer, which the RLV controller  804  may use to control or limit the amount of time that the RLV  800  adds or removes air to or from the one or more air springs  870  to an interval defined by the timer. In this way, the RLV controller  804  may use the timer to control or limit the amount of air added to or removed from the one or more air springs  870  in response to input received via the RLV user interface  802  and/or the communication interface  803 . In some alternative embodiments, the RLV controller  804  may include a raise timer and a lower timer. In these alternative embodiments, the raise timer may define an interval of time for supplying air to the one or more air springs  870  in response to a raise input (e.g., indicating that the user would like to raise the height of a side of the chassis of the vehicle  10  or trailer  20 ), and the lower timer may define an interval of time for removing air from the one or more air springs  870  in response to a lower input (e.g., indicating that the user would like to lower the height of a side of the chassis of the vehicle  10  or trailer  20 ). In these alternative embodiments, the intervals of time defined by the raise and lower timers may be different. In some aspects, the timer(s) may be set to an amount of time that it would take for the RLV  800  to completely raise and/or lower the height of the chassis to a fully raised or fully lowered height. However, this is not required, and, in some alternative aspects, the timer(s) may be set a smaller amount of time so that multiple raise or lower inputs would be required to reach the fully raised or fully lowered height. In this alternative aspects, a counter may be used to limit the number of consecutive raise or lower steps so that the chassis does not exceed the fully raised height or go below the fully lowered height. 
       FIG.  9    is a flow chart illustrating a raise lower process  900  according to some non-limiting aspects of the invention. In some aspects, the RLV  800  (e.g., the RLV controller  804 ) may perform one or more steps of the process  900 . 
     In some aspects, the process  900  may include a step  902  in which the when RLV  800  is not being used to raise or lower the height of the chassis of the vehicle  10  or trailer  20  (e.g., while the vehicle  10  or trailer  20  is traveling). In some aspects, in step  902 , the RLV controller  804  may control the first and second RLV actuators  806  and  808  to maintain the seals  842 ,  844 ,  846 , and  848  of the first and second RLV housings  814  and  816  in their neutral positions as shown in  FIG.  8 A  (e.g., by providing current to none of the solenoids  810   a ,  810   b ,  812   a , and  812   b ). In some aspects, with the seals  842 ,  844 ,  846 , and  848  of the first and second RLV housings  814  and  816  in their neutral positions as shown in  FIG.  8 A , the RLV  800  may allow normal operation of the leveling valve  860  in which the leveling valve  860  controls the height of a side of a chassis of the vehicle  10  or trailer  20  (e.g., to maintain a level condition of the vehicle  10  or trailer  20  while traveling). 
     In some aspects, when the first and second supply seals  842  and  844  of the first RLV housings  814  are in the neutral position as shown in  FIG.  8 A , the supply port  818  of the first RLV housing  814  may be connected pneumatically to the supply port  862   a  of the leveling valve  860  (e.g., via the supply passage  826  and leveling valve supply passage  828 ) such that the first RLV housing  814  supplies air received at the supply port  818  of the first RLV housing  814  to the supply port  862   a  of the leveling valve  860 . In some aspects, when in the neutral position as shown in  FIG.  8 A , the first supply seal  842  of the first RLV housing  814  may block air received at the supply port  818  from entering the leveling valve bypass passage  830  and the exhaust passage  832 . 
     In some aspects, when the first and second delivery seals  846  and  848  of the second RLV housing  816  are in the neutral position as shown in  FIG.  8 A , the spring port  862   d  of the leveling valve  860  may be connected pneumatically to the spring port  822  of the second RLV housing  816  (via the leveling valve spring passage  838  and the spring passage  840 ) such that the spring port  862   d  of the leveling valve  860  is pneumatically connected to the one or more air springs  870 , and the spring port  862   d  receives air from or supplies air to the one or more air springs  870 . In some aspects, when in the neutral position as shown in  FIG.  8 A , the first delivery seal  846  of the second RLV housing  816  may block air from entering the leveling valve bypass passage  836 . 
     In some aspects, in step  902 , with the seals  842 ,  844 ,  846 , and  848  in the neutral positions as shown in  FIG.  8 A , the leveling valve  860  can supply air to the one or more air springs  870  (e.g., by supplying air received at the supply port  862   a  of the leveling valve  860  to the one or more air springs  870  via the spring port  862   d  of the leveling valve  860 ) or remove air from the one or more air springs  870  (e.g., by receiving air from the one or more air springs  870  via the spring port  862   d  of the leveling valve  860  and venting the received air to the atmosphere via the exhaust port  862   b  of the leveling valve  860 ) as the leveling valve  860  deems appropriate to maintain the vehicle  10  or trailer  20  in a level condition. 
     In some aspects, the process  900  may include a step  904  in which the RLV  800  (e.g., the RLV controller  804 ) determines whether the RLV  800  (e.g., the RLV controller  804 ) has received via the RLV user input  802  and/or the communication interface  803  a raise input (e.g., indicating that the user would like to raise the height of a side of the chassis of the vehicle  10  or trailer  20  relative to the ground). In some aspects, the raise input may be, for example and without limitation, indicative of a switch of the RLV user interface  802  turned to the right. In some aspects, if the RLV  800  determines that no raise input was received in step  904 , the process  900  may proceed to a step  906 . In some aspects, if the RLV  800  determines that a raise input was received in step  904 , the process  900  may proceed to a step  908 . 
     In some aspects, the process  900  may include a step  906  in which the RLV  800  (e.g., the RLV controller  804 ) determines whether the RLV  800  (e.g., the RLV controller  804 ) has received via the RLV user input  802  and/or the communication interface  803  a lower input (e.g., indicating that the user would like to lower the height of a side of the chassis of the vehicle  10  or trailer  20  relative to the ground). In some aspects, the lower input may be, for example and without limitation, indicative of a switch of the RLV user interface  802  turned to the left. In some aspects, if the RLV  800  determines that no lower input was received in step  906 , the process  900  may proceed back to the step  902  in which the RLV  800  maintains the seals  842 ,  844 ,  846 , and  848  in the neutral positions so that the leveling valve  860  performs height control. In some aspects, if the RLV  800  determines that a raise input was received in step  904 , the process  900  may proceed to a step  918 . 
     In some aspects, the process  900  may include a step  908  in which the RLV controller  804 , in response to a raise input received via the RLV user interface  802  and/or the communication interface  803 , starts a timer of the RLV controller  804 . In some aspects, if the RLV controller  804  includes a raise timer and a lower timer, in step  908 , the RLV controller  804  may start the raise timer. 
     In some aspects, the process  900  may include a step  910  in which the RLV controller  804 , in response to a raise input, controls the first RLV actuator  806  to move the first and second supply seals  842  and  844  from the neutral position shown in  FIG.  8 A  to the raise position shown in  FIG.  8 B  (e.g., by passing an electrical current through one of the first and second solenoids  810   a  and  810   b  of the first RLV actuator  806 ) and controls the second RLV actuator  808  to move the first and second delivery seals  846  and  848  from the neutral position shown in  FIG.  8 A  to the raise position shown in  FIG.  8 B  (e.g., by passing an electrical current through one of the first and second solenoids  812   a  and  812   b  of the second RLV actuator  808 ). 
     In some aspects, in step  910 , when the first and second supply seals  842  and  844  of the first RLV housing  814  and the first and second delivery seals  846  and  848  of the second RLV housing  816  are in the raise positions, as shown in  FIG.  8 B , the RLV  800  may bypass the leveling valve  860  and supply air received at the supply port  818  of the first RLV housing  814  to the one or more air springs  870  (e.g., to increase the amount of air in the one or more air springs  870  and, thereby, increase the height of the chassis on a side of the vehicle  10  or trailer  20 ). In some aspects, with the seals  842 ,  844 ,  846 , and  848  in the raise positions, the RLV  800  may prevent the leveling valve  860  from controlling the height of a side of a chassis of the vehicle  10  or trailer  20 . 
     In some aspects, in step  910 , when the seals  842 ,  844 ,  846 , and  848  of the first and second RLV housings  814  and  816  are in the raise position as shown in  FIG.  8 B , the supply port  818  of the first RLV housing  814  may be connected pneumatically to the spring port  822  of the second RLV housing  816  (e.g., via the supply passage  826  of the first RLV housing  814 , the bypass path, and the spring passage  840  of the second RLV housing  816 ) such that the first and second RLV housings  814  and  816  supply air received at the supply port  818  of the first RLV housing  814  to the spring port  822  of the second RLV housing  816 . In some aspects, the bypass path may include the leveling valve bypass passage  830  of the first RLV housing  814 , the bypass air line  850 , and the leveling valve bypass passage  836  of the second RLV housing  816 . In some aspects, when in the raise position as shown in  FIG.  8 B , the second supply seal  844  of the first RLV housing  814  may block air received at the supply port  818  from entering the leveling valve supply passage  828  and, thus, prevent air received at the supply port  818  from reaching the supply port  862   a  of the leveling valve  860 . In some aspects, when in the raise position as shown in  FIG.  8 B , the second delivery seal  848  of the second RLV housing  816  may block the leveling valve spring passage  838  and, thus, prevent air in the bypass path from reaching the spring port  862   d  of the leveling valve  860 . In some aspects, after the RLV  800  has added air to the one or more air springs  870  and increased the height of the chassis on one side of the vehicle  10  or trailer  20 , the leveling valve  860  (if operational) would attempt to remove air from the one or more air springs  870  to return the vehicle  10  or trailer  20  to a level condition, but the second delivery seal  848  in the raise position prevents the leveling valve  860  from operating to remove air from the one or more air springs  870 . 
     In some aspects, the process  900  may include a step  912  in which the RLV controller  804  determines whether the timer (e.g., the raise timer) has expired. In some aspects, the timer may be set to expire after an amount of time allows for enough air to be added to the one or more air springs  870  that the height of the side of the vehicle  10  or trailer  20  is raised relative to the ground by a certain amount (e.g., 6 inches or 1 foot). In some aspects, the step  912  may additionally or alternatively include the RLV controller  804   a  determining that the air pressure within the one or more air springs has reached a threshold raise pressure (e.g., using a pressure sensor such as pressure sensor  120 ,  122 ,  220 , or  222 ) and/or that the side of the chassis of the vehicle  10  or trailer  20  has reached a threshold raise height (e.g., using a proximity sensor). In some aspects, if the RLV controller  804  determines that the timer has expired (or that the threshold raise pressure and/or threshold raise height has been reached) in step  912 , the process  900  may proceed to a step  914 . In some aspects, if the RLV controller  804  determines that the timer has not expired (or that the threshold raise pressure and/or threshold raise height have not been reached) in step  912 , the process  900  may proceed back to the step  910  so that the RLV  800  continues to add air to the one or more air springs  870  to raise the height of the side of the chassis of the vehicle  10  or trailer  20  relative to the ground. 
     In some aspects, the process  900  may include a step  914  in which the RLV controller  804 , in response to the interval of time having passed (or the threshold raise pressure and/or threshold raise height having been reached), controls the first RLV actuator  806  to move the first and second supply seals  842  and  844  from the raise position shown in  FIG.  8 B  to the neutral position shown in  FIG.  8 C  (e.g., by providing no electrical current to the first and second solenoids  810   a  and  810   b  of the first RLV actuator  806 ) and controls the second RLV actuator  808  to maintain the first and second delivery seals  846  and  848  in the raise position shown in  FIG.  8 C  (e.g., by continuing to pass the electrical current through one of the first and second solenoids  812   a  and  812   b  of the second RLV actuator  808 ). In some aspects, moving the supply seals  842  and  844  to the neutral position and keeping the delivery seals  846  and  848  in the raise position will maintain the increased amount of air in the one or more air springs  870  and maintain the raised height of the one side of the chassis of the vehicle  10  or the trailer  20 . 
     In some aspects, in step  914 , when the first and second supply seals  842  and  844  of the first RLV housings  814  are in the neutral position as shown in  FIG.  8 C , the first supply seal  842  of the first RLV housing  814  may block pneumatic communication of the leveling valve bypass passage  830  of the bypass path with the exhaust passage  832  and pneumatic communication of the supply passage  826  with the leveling valve bypass passage  830 . In some aspects, when the first and second delivery seals  846  and  848  of the second RLV housing  816  are in the raise position as shown in  FIG.  8 C , the second delivery seal  848  of the second RLV housing  816  may block the leveling valve spring passage  838  and prevent pneumatic communication of the spring passage  840  with the spring port  862   d  of the leveling valve  860 . Accordingly, with the supply seals  842  and  844  in the neutral position and the delivery seals  846  and  848  in the raise position, the raised height of the side of the chassis of the vehicle  10  or trailer  20  is maintained because (i) the first supply seal  842  prevents additional air received at the supply port  818  from entering the leveling valve bypass passage  830 , (ii) the first supply seal  842  prevents air from the one or more air springs  870  from being vented to atmosphere via the exhaust passage  832  and exhaust port  820 , and (iii) the second delivery seal  848  prevents air from the one or more air springs  870  from being vented to atmosphere via the leveling valve  860  (e.g., via the spring port  862   d  and then through the exhaust port  862   b  of the leveling valve). With the supply seals  842  and  844  in the neutral position, the supply port  818  of the first RLV housing  814  may be connected pneumatically to the supply port  862   a  of the leveling valve  860  (e.g., via the supply passage  826  and leveling valve supply passage  828 ) such that the first RLV housing  814  could supply air received at the supply port  818  of the first RLV housing  814  to the supply port  862   a  of the leveling valve  860 . However, as the side of the chassis of the vehicle  10  or trailer  20  is at a raised height in step  914 , the leveling valve  860  would be trying to remove air from the one or more air bags  870  (if the ability to do so were not blocked by the second delivery seal  848 ) and would not be trying to supply more air to the one or more air springs  870 . 
     In some aspects, the process  900  may include a step  916  in which the RLV  800  (e.g., the RLV controller  804 ) determines whether the RLV  800  (e.g., the RLV controller  804 ) has received via the RLV user input  802  and/or the communication interface  803  a leveling valve height control input (e.g., indicating that the user would like to return height control to the leveling valve  860 , which will return the side of the chassis of the vehicle  10  or trailer  20  to the normal ride height). In some aspects, the leveling valve height control may be indicative of, for example and without limitation, a switch of the RLV user interface  802  turned to the center position. In some aspects, if the RLV  800  determines that no leveling valve height control input was received in step  916  (e.g., determines that a switch of the RLV user interface  802  was kept in the raise position and was not moved to the center position and/or that no leveling valve height control input was received via the communication interface  803 ), the process  900  may proceed back to the step  914  in which the RLV  800  maintains the supply seals  842  and  844  in the neutral position and the delivery seals  846  and  848  in the raise position so that the side of the chassis of the vehicle  10  or trailer  20  is maintained at the raised height. In some aspects, if the RLV  800  determines that a leveling valve height control input was received in step  916 , the process  900  may proceed to the step  902  in which the seals  842 ,  844 ,  846 , and  848  are all moved to their neutral positions so that the leveling valve  860  performs height control, which will return (e.g., lower) the height of the side of the chassis of the vehicle  10  or trailer  20  to its normal ride height. 
     In some aspects, the process  900  may include a step  918  in which the RLV controller  804 , in response to a lower input received via the RLV user interface  802  and/or the communication interface  803 , starts a timer of the RLV controller  804 . In some aspects, if the RLV controller  804  includes a raise timer and a lower timer, in step  918 , the RLV controller  804  may start the lower timer. 
     In some aspects, the process  900  may include a step  920  in which the RLV controller  804 , in response to a lower input, controls the first RLV actuator  806  to move the first and second supply seals  842  and  844  from the neutral position shown in  FIG.  8 A  to the lower position shown in  FIG.  8 D  (e.g., by passing an electrical current through one of the first and second solenoids  810   a  and  810   b  of the first RLV actuator  806 ) and controls the second RLV actuator  808  to move the first and second delivery seals  846  and  848  from the neutral position shown in  FIG.  8 A  to the lower position shown in  FIG.  8 D  (e.g., by passing an electrical current through one of the first and second solenoids  812   a  and  812   b  of the second RLV actuator  808 ). 
     In some aspects, in step  920 , when the first and second supply seals  842  and  844  of the first RLV housing  814  and the first and second delivery seals  846  and  848  of the second RLV housing  816  are in the lower positions, as shown in  FIG.  8 D , the RLV  800  may bypass the leveling valve  860  and provide air from one or more air springs  870  to the exhaust port  820  of the first RLV housing  814  (e.g., to decrease the amount of air in the one or more air springs  870  and, thereby, decrease the height of the side of the chassis of the vehicle  10  or trailer  20 ). In some aspects, with the seals  842 ,  844 ,  846 , and  848  in the lower positions, the RLV  800  may prevent the leveling valve  860  from controlling the height of a side of a chassis of the vehicle  10  or trailer  20 . 
     In some aspects, in step  920 , when the seals  842 ,  844 ,  846 , and  848  of the first and second RLV housings  814  and  816  are in the lower position as shown in  FIG.  8 D , the spring port  822  of the second RLV housing  816  may be connected pneumatically to the exhaust port  820  of the first RLV housing  814  (e.g., via the spring passage  840  of the second RLV housing  816 , bypass path, and exhaust passage  832  of the first RLV housing  814 ) such that the first and second RLV housings  814  and  816  provide air received at the spring port  822  of the second RLV housing  816  to the exhaust port  820  of the first RLV housing  814 . In some aspects, the bypass path may include the leveling valve bypass passage  830  of the first RLV housing  814 , the bypass air line  850 , and the leveling valve bypass passage  836  of the second RLV housing  816 . In some aspects, when in the lower position as shown in  FIG.  8 D , the first delivery seal  846  and/or second delivery seal  848  of the second RLV housing  816  may block the leveling valve spring passage  838  and, thus, prevent pneumatic communication between (i) the spring port  862   d  of the leveling valve  860  and (ii) the spring port  822  of the second RLV housing  816  and/or the bypass path. In some aspects, when in the lower position as shown in  FIG.  8 D , the first supply seal  842  and/or the second supply seal  844  of the first RLV housing  814  may block the leveling valve supply passage  828  and, thus, prevent pneumatic communication between (i) the supply port  862   a  of the leveling valve  860  and (ii) the bypass path and/or the exhaust port  820  of the first RLV housing  814 . In some aspects, when in the lower position as shown in  FIG.  8 D , the first supply seal  842  and/or the second supply seal  844  of the first RLV housing  814  may block the supply passage  826  and, thus, prevent pneumatic communication between (i) the supply port  818  of the first RLV housing  814  and (ii) the supply port  862   a  of the leveling valve  860  and/or the bypass path. In some aspects, after the RLV  800  has removed air from the one or more air springs  870  and decreased the height of the chassis on one side of the vehicle  10  or trailer  20 , the leveling valve  860  (if able to operate) would attempt to add air to the one or more air springs  870  to return the vehicle  10  or trailer  20  to a level condition, but the seals  842 ,  844 ,  846 , and  848  in the lower positions prevent the leveling valve  860  from operating to add air to the one or more air springs  870 . 
     In some aspects, the process  900  may include a step  922  in which the RLV controller  804  determines whether the timer (e.g., the lower timer) has expired. In some aspects, the timer may be set to expire after an amount of time allows for enough air to be removed from the one or more air springs  870  that the height of the side of the vehicle  10  or trailer  20  is lowered relative to the ground by a certain amount (e.g., 6 inches or 1 foot). In some aspects, the step  922  may additionally or alternatively include the RLV controller  804   a  determining that the air pressure within the one or more air springs has fallen to a threshold lower pressure (e.g., using a pressure sensor such as pressure sensor  120 ,  122 ,  220 , or  222 ) and/or that the side of the chassis of the vehicle  10  or trailer  20  has fallen to a threshold lower height (e.g., using a proximity sensor). In some aspects, if the RLV controller  804  determines that the timer has expired (or that the threshold lower pressure and/or threshold lower height has been reached) in step  922 , the process  900  may proceed to a step  924 . In some aspects, if the RLV controller  804  determines that the timer has not expired (or that the threshold raise pressure and/or threshold raise height have not been reached) in step  922 , the process  900  may proceed back to the step  920  so that the RLV  800  continues to remove air from the one or more air springs  870  to lower the height of the side of the chassis of the vehicle  10  or trailer  20  relative to the ground. 
     In some aspects, the process  900  may include a step  924  in which the RLV controller  804 , in response to the interval of time having passed (or the threshold lower pressure and/or threshold lower height having been reached), controls the first RLV actuator  806  to maintain the first and second supply seals  842  and  844  in the lower position shown in  FIG.  8 E  (e.g., by continuing to pass the electrical current through one of the first and second solenoids  810   a  and  810   b  of the first RLV actuator  806 ) and controls the second RLV actuator  808  to move the first and second delivery seals  846  and  848  from the lower position to the neutral position shown in  FIG.  8 E  (e.g., by providing no electrical current to the first and second solenoids  812   a  and  812   b  of the second RLV actuator  808 ). In some aspects, keeping the supply seals  842  and  844  in the lower position and moving the delivery seals  846  and  848  to the neutral position will maintain the decreased amount of air in the one or more air springs  870  and maintain the lowered height of the one side of the chassis of the vehicle  10  or the trailer  20 . 
     In some aspects, in step  924 , when the first and second delivery seals  846  and  848  of the second RLV housings  816  are in the neutral position as shown in  FIG.  8 E , the first delivery seal  846  may block pneumatic communication of the spring passage  840  with the leveling valve bypass passage  830  of the bypass path. In some aspects, when the first and second supply seals  842  and  844  of the first RLV housing  814  are in the lower position as shown in  FIG.  8 E , the first and/or second supply seals  842  and  844  may block the supply passage  826  and the leveling valve supply passage  828  and prevent pneumatic communication of the supply passage  826  with the supply port  862   a  of the leveling valve  860 . Accordingly, with the supply seals  842  and  844  in the lower position and the delivery seals  846  and  848  in the neutral position, the lowered height of the side of the chassis of the vehicle  10  or trailer  20  is maintained because (i) the first supply seal  842  prevents air received at the supply port  818  from entering the leveling valve bypass passage  830 , (ii) the first delivery seal  846  prevents air from the one or more air springs  870  from being vented to atmosphere via the bypass path (e.g., including the leveling valve bypass passage  836  of the second RLV housing  816 , the bypass air line  850 , and the leveling valve bypass passage  830  of the first RLV housing  814 ), exhaust passage  832 , and exhaust port  820 , and (iii) the first and/or second supply seals  842  and  844  prevent air from supply port  818  of the first RLV housing  814  being supplied to the supply port  862   a  of the leveling valve  860  (e.g., via the leveling valve supply passage  828 ). With the delivery seals  846  and  848  in the neutral position, the spring port  822  of the second RLV housing  816  may be connected pneumatically to the spring port  862   d  of the leveling valve  860  (e.g., via the spring passage  840  and the leveling valve spring passage  838 ) such that the second RLV housing  816  could provide air received at the spring port  822  of the second RLV housing  816  to the spring port  862   d  of the leveling valve  860  for venting to atmosphere via the exhaust port  862   b  of the leveling valve  860 . However, as the side of the chassis of the vehicle  10  or trailer  20  is at a lowered height in step  924 , the leveling valve  860  would be trying to supply air to the one or more air bags  870  (if the ability to do so were not blocked by the first and second supply seals  842  and  844 ) and would not be trying to remove more air from the one or more air springs  870 . 
     In some aspects, the process  900  may include a step  926  in which the RLV  800  (e.g., the RLV controller  804 ) determines whether the RLV  800  (e.g., the RLV controller  804 ) has received via the RLV user input  802  and/or the communication interface  803  a leveling valve height control input (e.g., indicating that the user would like to return height control to the leveling valve  860 , which will return the side of the chassis of the vehicle  10  or trailer  20  to the normal ride height). In some aspects, the leveling valve height control input may be, for example and without limitation, be indicative of a switch of the RLV user input  802  turned to the center position. In some aspects, if the RLV  800  determines that no leveling valve height control input was received in step  926  (e.g., determines that a switch of the RLV user interface  802  was kept in the lower position and was not moved to the center position and/or that no leveling valve height control input was received via the communication interface  803 ), the process  900  may proceed back to the step  924  in which the RLV  800  maintains the supply seals  842  and  844  in the lower position and the delivery seals  846  and  848  in the neutral position so that the side of the chassis of the vehicle  10  or trailer  20  is maintained at the lowered height. In some aspects, if the RLV  800  determines that a leveling valve height control input was received in step  926 , the process  900  may proceed to the step  902  in which the seals  842 ,  844 ,  846 , and  848  are all moved to their neutral positions so that the leveling valve  860  performs height control, which will return (e.g., raise) the height of the side of the chassis of the vehicle  10  or trailer  20  to its normal ride height. 
     Although in some aspects the RLV  800  uses a timer (or timers) to limit the amount of air the RLV  800  adds to or removes from the one or more air springs  870 , this is not required. In some alternative aspects, the RLV  800  may not use a timer or timer and instead uses one or more sensors (e.g., one or more pressure sensors and/or one or more proximity sensors) to determine the appropriate time to stop raising or lowering the height of the side of the chassis of the vehicle  10  or trailer  20 . In some further alternative aspects, the RLV  800  may not limit the amount of air the RLV  800  adds to or removes from the one or more air springs  870 . In these alternative embodiments, the controller  804  may control the first and second RLV actuators  806  and  808  to move the seals  842 ,  844 ,  846 , and  848  to the raise or lower positions (e.g., as in  FIG.  8 B or  8 D ) so that the RLV  800  adds or removes air to or from the one or more air springs  870  the entire time that the raise or lower input is received via the RLV user interface  802  and/or the communication interface  803  (e.g., indicating that the user would like to raise or lower the height of a side of the chassis of the vehicle  10  or trailer  20  relative to the ground). For example, in some aspects, the RLV  800  may add or remove air to or from the one or more air springs  870  the entire time that a switch of the RLV user interface  802  is turned to the right or left. 
     In some aspects, the RLV controller the RLV user interface  802 , the communication interface  803 , and/or RLV controller  804  may be contained in the same housing. In some aspects, the housing including the RLV user interface  802 , the communication interface  803 , and/or RLV controller  804  may be located in a cabin of the vehicle  10 , at a back corner of the trailer  20 , or in a control box. 
     In some aspects, as shown in  FIG.  10   , the RLV user interface  802 , the communication interface  803 , and/or the RLV controller  804  may be used to control raising or lowering of the height on either or both sides of the chassis of a vehicle  10  or trailer  20 . In some aspects, as shown in  FIG.  10   , the supply and spring ports  862   a  and  862   d  of a first leveling valve  860  (e.g., first leveling valve  108  or  208 ) on the right side of the vehicle  10  or trailer  20  may be connected pneumatically to the main passages  824  and  834 , respectively, of a first set of first and second RLV housings  814  and  816 , and supply and spring ports  862   a  and  862   d  of a second leveling valve  860  (e.g., first leveling valve  112  or  212 ) on the left side of the vehicle  10  or trailer  20  may be connected pneumatically to the main passages  824  and  834 , respectively, of a second set of first and second RLV housings  814  and  816 . In some aspects, the RLV user interface  802  may include a first user input (e.g., a three-position switch) for providing raise, leveling valve height control, or lower inputs (e.g., right, center, and left positions, respectively, or upper, center, and lower positions, respectively) and a second user input (e.g., a three-position switch) for providing right side, both sides, or left side inputs (e.g., right, center, and left positions, respectively, or upper, center, and lower positions, respectively). In some aspects, the raise, leveling valve height control, and lower inputs and/or right side, both sides, and left side inputs may additionally or alternatively be provided to the RLV controller  804  via the communication interface  803 . 
     In some aspects, if the RLV controller  804  receives the both sides input from the second user input (or from the communication interface  803 ), then the RLV controller  804  may control the actuators  806  and  808  for both sets of first and second RLV housings  814  and  816  in accordance with the process  900  shown in  FIG.  9    in response to raise, leveling valve height control, and lower inputs from the first user input (or from the communication interface  803 ) (e.g., so that the height of chassis of the vehicle  10  or trailer  20  on both the right and lefts sides is raised, controlled by the respective leveling valve  860 , or lowered). In some aspects, if the RLV controller  804  receives the right side input or the left side input from the second user input (or from the communication interface  803 ), then the RLV controller  804  may (i) control the actuators  806  and  808  for the set of first and second RLV housings  814  and  816  on the selected side in accordance with the process  900  shown in  FIG.  9    in response to raise, leveling valve height control, and lower inputs from the first user input (or from the communication interface  803 ) and (ii) control the actuators  806  and  808  for the first and second RLV housings  814  and  816  on the non-selected side maintain the amount of air in the air springs  870  on the non-selected side and, therefore, maintain the height of the chassis on the non-selected side. In some aspects, if no action were taken on the non-selected side (e.g., if the seals  842 ,  844 ,  846 , and  848  of the RLV housings  814  and  816  on the non-selected side were kept in the neutral positions) while raising or lowering the selected side, the leveling valve  860  on the non-selected side would attempt to compensate for the height change on the selected side. Accordingly, in some aspects, in response to a raise input for a selected side, the RLV controller  804  may control the actuators  806  and  808  on the non-selected side to move the supply seals  842  and  844  from the neutral position to the lower position and to maintain the delivery seals  846  and  848  in the neutral position (resulting in the arrangement shown in  FIG.  8 E ). When arranged as shown in  FIG.  8 E , the seals  842 ,  844 ,  846 , and  848  on the non-selected side may maintain the amount of air in the one or more air springs  870  of the non-selected side and prevent the leveling valve  860  on the non-selected side from adding air to the one or more air springs  870  on the non-selected side to match the air added to the one or more air springs  870  on the selected side. In addition, in some aspects, in response to a lower input for a selected side, the RLV controller  804  may control the actuators  806  and  808  on the non-selected side to maintain the supply seals  842  and  844  in the neutral position and to move the delivery seals  846  and  848  from the neutral position to the raise position (resulting in the arrangement shown in  FIG.  8 C ). When arranged as shown in  FIG.  8 C , the seals  842 ,  844 ,  846 , and  848  on the non-selected side may maintain the amount of air in the one or more air springs  870  of the non-selected side and prevent the leveling valve  860  on the non-selected side from removing air from the one or more air springs  870  on the non-selected side to match the air removed from the one or more air springs  870  on the selected side. 
     In some alternative aspects, the RLV user interface  802  may include a first user input (e.g., a three-position switch) for providing raise, leveling valve height control, or lower inputs (e.g., right, center, and left positions, respectively, or upper, center, and lower positions, respectively) for controlling the height on one side of the vehicle  10  or trailer  20 , and a second user input (e.g., a three-position switch) for providing raise, leveling valve height control, or lower inputs (e.g., right, center, and left positions, respectively, or upper, center, and lower positions, respectively) for controlling the height on the other side of the vehicle  10  or trailer  20 . 
     In some alternative embodiments, the user input that provides the raise, neutral, or lower inputs may be a variable switch (e.g., a potentiometer type switch) and allow for one up and one down at the same time as required. 
     In some aspects, the cross-flow passage  864  (e.g., the cross-flow passage  116  or  216 ) may be configured to connect the leveling valves  860  on opposite sides of the vehicle  10  or trailer  20 . In some aspects, the leveling valves  860  may be configured to establish pneumatic communication via the cross-flow passage  864  when the leveling valves  860  are allowed to adjust independently the heights of the first and second sides of the vehicle or the trailer (e.g., because the RLV controller  804  has controlled the seals  842 ,  844 ,  846 , and  848  of the housings  814  and  816  on both sides to the neutral positions) but neither of the leveling valves  860  is adjusting independently the height of a side of the vehicle  10  or trailer  20 . In some aspects, the pneumatic communication between the pneumatic circuits via the cross-flow passage  864  may equalize air pressure between air springs  870  on opposite sides of the vehicle  10  or trailer  20 . 
     Similar to the computer  124  of the vehicle system  100 , in some aspects, as shown in  FIG.  4   , the RLV controller  804  may include one or more processors  522  (e.g., a general purpose microprocessor) and/or one or more circuits, such as an application specific integrated circuit (ASIC), field-programmable gate arrays (FPGAs), a logic circuit, and the like. In some aspects, the RLV controller  804  may include a data storage system (DSS)  523 . The DSS  523  may include one or more non-volatile storage devices and/or one or more volatile storage devices (e.g., random access memory (RANI)). In aspects where the RLV controller  804  includes a processor  522 , the DSS  523  may include a computer program product (CPP)  524 . CPP  524  may include or be a computer readable medium (CRM)  526 . The CRM  526  may store a computer program (CP)  528  comprising computer readable instructions (CRI)  530 . The CRM  526  may be a non-transitory computer readable medium, such as, but not limited, to magnetic media (e.g., a hard disk), optical media (e.g., a DVD), solid state devices (e.g., random access memory (RAM) or flash memory), and the like. In some aspects, the CRI  530  of computer program  528  may be configured such that when executed by processor  522 , the CRI  530  causes the computer  124  to perform one or more of the steps described with reference to the RLV  800  (e.g., the steps of the process  900  shown in  FIG.  9   ). In other aspects, the RLV controller  804  may be configured to perform steps described herein without the need for a computer program. That is, for example, the computer may consist merely of one or more ASICs. Hence, the features of the aspects described herein may be implemented in hardware and/or software. 
     In any of the aspects above, the vehicle  10  and/or the trailer  20  may be an autonomous vehicle, and the vehicle  10  and/or trailer  20  may be capable of operating without input from a user/driver. In some aspects, the vehicle  10  and/or the trailer  20  may be a self-driving vehicle. In some aspects, the computer  124  and/or the RLV controller  804  may be configured for autonomous operation (e.g., for operation without input from a user/driver). In some aspects, autonomous operation may include communication (e.g., wireless communication) with one or more other vehicles or trailers (e.g., using the communication unit  139  and/or the communication interface  803 ) and/or with one or more other devices (a communication unit at a loading dock and/or another computer such as a smartphone). For example, in some aspects, the computer  124  and/or the RLV controller  804  may, without user input, may control the RLV  800  to raise or lower one or both sides of the vehicle  10  or the trailer  20  (e.g., as appropriate for unloading or loading to or from a loading dock of a particular height). 
     In any of the aspects described above, the leveling valves may be the same or similar to leveling valves described in U.S. Pat. No. 10,093,145, which is incorporated by reference herein in its entirety. 
     Aspects of the present invention have been fully described above with reference to the drawing. Although the invention has been described based upon these preferred aspects, it would be apparent to those of skill in the art that certain modifications, variations, and alternative constructions could be made to the described aspects within the spirit and scope of the invention.