Abstract:
A supplemental braking system for use on a towed vehicle being towed behind a towing vehicle, especially an automobile being towed behind a motor coach by a tow bar. The supplemental braking system utilizes a vacuum motor to depress the brake pedal in the towed vehicle and thereby apply the brakes on the towed vehicle. The vacuum motor is powered by a vacuum supplied by either the natural vacuum created by the intake of a gas engine of the towing vehicle, or by a separate vacuum pump mounted on the towing vehicle. The same vacuum that is used to provide power to the vacuum motor is also used to supply a vacuum to the vacuum booster on the power brake system of the towed vehicle, thereby permitting much smaller pulling forces to be used in applying the brakes of the towed vehicle. Control valves are provided to selectively apply the brakes of the towed vehicle when the brakes of the towing vehicle are applied. The control valves can be set to only partially apply the brakes on the towed vehicle in the standard braking situation, but to fully apply the brakes in instances where the driver of the towing vehicle is initiating an emergency, or panic, stop or in instances where the towed vehicle has broken away from the towing vehicle.

Description:
FIELD OF INVENTION  
         [0001]    This invention relates generally to supplemental braking systems for towed vehicles, and more specifically to an apparatus that utilizes a vacuum motor having a flexible membrane with a mechanical connection to the towed vehicle&#39;s brake pedal, to depress the towed vehicle&#39;s brake pedal and thereby apply the towed vehicle&#39;s brakes.  
         BACKGROUND OF THE INVENTION  
         [0002]    Recreational vehicles or motor coaches, are a common mode of transportation that are increasing in popularity. Because the motor coaches tend to be large and cumbersome, it is common practice to tow an automobile such as a passenger car, behind the motor coach to use once the motor coach is parked for the evening. However, the brakes on the motor coach are not necessarily large enough to safely stop the motor coach with the added weight of the towed vehicle. For this reason, it is known to provide a supplemental braking system for use with the towed vehicle.  
           [0003]    The supplemental braking systems have taken several different forms. One common supplemental braking system employs compressed air to extend a rod in a pneumatic cylinder to push the towed vehicle&#39;s brake pedal and thereby employ the towed vehicle&#39;s braking system. An example of this type of system is shown in Wittkop et al., U.S. Pat. No. 5,031,729. A disadvantage of these types of systems is that they do not take advantage of the power booster that is included on most automobile braking systems. Therefore, the supplemental braking systems that utilize compressed air with pneumatic pistons, typically must press much harder on the brake pedal than is required when the towed vehicle is operational. This creates unnecessary stress and fatigue on the brake components. Furthermore, the compressed air-type supplemental braking systems tend to apply the brakes on a towed vehicle quite abruptly.  
           [0004]    Another common type of supplemental braking system is a serge brake system. A serge brake system uses the relative pushing force of the momentum of the towed vehicle against the towing vehicle as the towing vehicle slows to apply the brakes of the towed vehicle. A disadvantage of these type of systems is that they do not apply the towed vehicle&#39;s brakes until after the towing vehicle has slowed enough that the momentum of the towed vehicle begins to push the towing vehicle. It is common for these serge brake type systems to be attached directly into the master cylinder of the towed vehicle. This presents a problem because it can avoid the warranty on the braking system of the towed vehicle.  
           [0005]    A common method of towing a vehicle behind a motor coach is through the use of a tow bar. Tow bars are advantageous because they provide a simple, and relatively light weight method of attaching a towed vehicle behind a motor coach. When a tow bar is used, all four wheels of the towed vehicle remain on the pavement just as when in normal use. Therefore, the braking system of the towed vehicle can be utilized to help slow the towed vehicle.  
           [0006]    Most vehicles that would be towed behind a motor coach for use after the motor coach is parked are provided with power brakes. The most common power brake system in use today employs a vacuum booster. These vacuum boosters utilize the vacuum created by gasoline engines to amplify the force applied by the driver on the brake pedal. When the engine of the towed vehicle is not running, no vacuum is created, and therefore, the vacuum booster will not provide any amplification to the force applied to the brake pedal. The braking system of the towed vehicle typically will work when the vehicle is off, however, a much greater force must be applied to the brake pedal in order to be effective.  
           [0007]    A desirable feature for supplemental braking systems is to provide at least one intermediate level of braking that is less than a maximum application of the towed vehicle&#39;s brakes. If the brakes on the towed vehicle are applied with too much force, or to quickly, it can cause the towed vehicle to provide unwanted additional resistance to the motor coach, and will put undue stress on the brakes of the towed vehicle. Furthermore, it can cause the tires on the towed vehicle to lock and go into a skid. On the other hand, in some instances it is desired to fully apply the brakes of the towed vehicle, as for example, in an emergency stop when the driver is attempting to stop the motor coach and towed vehicle as quickly as possible.  
           [0008]    In rare instances, it is possible for a towed vehicle to become disengaged from the motor coach during towing. This can be a dangerous phenomenon as the towed vehicle can have a great deal of forward momentum, but will have no one steering it. This phenomenon is known commonly as a break away. It is desirable, and in some cases, required by law, for such a break away vehicle to have its brakes applied automatically to bring the break away vehicle to a quick stop.  
           [0009]    U.S. Pat. No. 6,152,544 describes a supplemental braking system that takes advantage of the vacuum booster of the brakes of the towed vehicle. This system utilizes a pneumatic piston that retracks a cable attached to the brake pedal of the towed vehicle to pull rather than push the brake pedal of the towed vehicle. Furthermore, it utilizes a remote vacuum source, either in the form of the natural vacuum created by the motor of the motor coach, or by a separate vacuum pump, to provide vacuum to the vacuum booster of the towed vehicle&#39;s braking system, and to provide the power for the pneumatic cylinder. This system provides only one level of braking power. Furthermore, pneumatic cylinders can be susceptible to leaks and other difficulties that are not desirable.  
           [0010]    The present invention overcomes many of the difficulties associated with the prior supplemental braking systems.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention is directed to an apparatus for applying the brakes on a towed vehicle being pulled by a towing vehicle. The towed vehicle being of the type having a self-contained braking system that is activated by depressing a brake pedal. The towing vehicle has a source for supplying a vacuum. A vacuum motor having a flexible membrane movable by application of a vacuum is connected to the vacuum source in the towing vehicle through a pneumatic circuit. The flexible membrane in the vacuum motor is mechanically connected with the brake pedal in the towed vehicle such that as the membrane moves from a rest position towards a fully activated position, through the application of a vacuum through the vacuum motor, the brake pedal in the towed vehicle is depressed, thereby applying the towed vehicle&#39;s brakes. The pneumatic circuit includes a control valve which has an activated position that completes a pneumatic circuit between the vacuum source in the towing vehicle and the vacuum motor. The control valve also has a deactivated position wherein the vacuum source is diverted from the vacuum motor. A means is provided for selectively moving the control valve between the activated and deactivated positions.  
           [0012]    The primary objective of the present invention is to provide a supplemental braking system for a towed vehicle that utilizes a vacuum motor with a flexible membrane to selectively apply the brakes of the towed vehicle. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a schematic of the pneumatic and electrical components of the present invention;  
         [0014]    [0014]FIG. 2 is a detailed drawing of a preferred embodiment of a vacuum motor for use in conjunction with the present invention;  
         [0015]    [0015]FIG. 3 is a detailed drawing showing the pneumatic components of a preferred embodiment of the present invention;  
         [0016]    [0016]FIG. 4 is a detailed view of a panic switch for use in association with the present invention  
         [0017]    [0017]FIG. 5 is a detailed view of panic switch of FIG. 4, as it is installed with a leveling device; and  
         [0018]    [0018]FIG. 6 is a detailed view of the mounting bracket, vacuum motor, and brake pedal pulling wire according to a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    Shown generally in the drawings is a system for applying the brakes on a towed vehicle  10  that is being pulled by a towing vehicle  12 . The system comprises mechanical and pneumatic components, including a vacuum motor  14  that is attached to a brake pedal  16  of the towed vehicle  10 , and electrical components. When the appropriate electrical signal is sent, the vacuum motor  14  pulls the brake pedal  16  to apply the brakes of the towed vehicle  10 . FIG. 1 shows a symbolic schematic of the electrical and pneumatic components of the system.  
         [0020]    Starting at the towing vehicle  12 , and working back towards the brake pedal  16 , the mechanical and pneumatic components can be described as follows. A vacuum source  18  is provided on the towing vehicle  12 . If the towing vehicle  12  has a gas engine, the vacuum source is preferably the intake manifold of the gas engine. If the towing vehicle has a diesel engine, a separate vacuum pump should be mounted in the towing vehicle. Typically a vacuum of about  21  inches of mercury is sufficient to provide the necessary vacuum force. An air hose  20   a  runs from the vacuum source  18  to the rear of the towing vehicle  12 , where it is preferably provided with a quick connect coupling  22   a . Another air hose  20   b  connects the quick connect coupling  22   a  at the rear of the towing vehicle with a quick connect coupling  22   b  at the front of the towed vehicle  10 .  
         [0021]    A pneumatic circuit  24  connects the quick connect coupling  22   b  at the front of the towed vehicle  10  with the vacuum motor  14 . The basic components of the pneumatic circuit can be seen in FIG. 3. An air hose connects the quick connect coupling  22   b  with the brake booster  26  of the towed vehicle  10 . In practice this is preferably done by removing the brake booster supply line of the towed vehicle  10  and replacing it with a T-connection  28 . This is done because some brake booster supply lines are provided with a check valve that can cause difficulties. A first check valve  30  is provided between the inlet port  22   b  and the T-connection so that air can flow from the brake booster  26  towards the vacuum source  18 , but cannot flow in the opposite direction. A vacuum reservoir  34 , typically an accumulator, is connected to the pneumatic circuit  24  on the vacuum motor side of the brake booster  26 . The vacuum reservoir  34  provides vacuum to the brake booster  26 , in case the pneumatic circuit  24  comes disconnected from the vacuum source  18 . A second check valve  32  is provided between the vacuum reservoir  34  and the brake booster  26 , which permits air to flow towards the vacuum source, but prevents air from flowing in the opposite direction.  
         [0022]    The pneumatic circuit  24  then runs through a control box  36 . The control box includes two control valves  38 ,  40  in series with each other. The first control valve  38  regulates the pressure level applied to the vacuum motor  14 . The second control valve  40  acts as an on-off valve. Both of the control valves  38 ,  40  are two-position, three-way valves that are normally in a deactivated position. When they are energized, they move to an activated position to complete the circuit so that air can flow from the vacuum motor  14  towards the vacuum source  18 . The first control valve  38  has its exhaust port closed, so that atmospheric pressure air cannot enter the system when the first control valve  38  is deactivated. The second control valve  40  has its exhaust port going to atmosphere, to permit atmospheric pressure air to enter the system when the second control valve  40  is deactivated. The control valves  38 ,  40  are controlled by a control switch  44 , that is also located in the control box  36 .  
         [0023]    The control box  36  is connected to the vacuum motor  14  with another air hose. A detailed view of the vacuum motor  14  is shown in FIG. 2. The vacuum motor  14  includes a rigid air tight outer shell  46  with an inlet port  48 . A flexible membrane  54  seals the vacuum motor interior into two portions, a vacuum side  50  and a brake side  52 . A mounting plate  56  on the vacuum side  50  of the flexible membrane receives a stem  58  that extends from rod  60 . Rod  60  is connected to cable  62 , which goes through the firewall  64  of the towed vehicle  10 . A vacuum motor  14  is believed to be superior to other methods of depressing the brake pedal  16 . Vacuum motors are robust and less likely to suffer leaks than air piston and cylinder arrangements.  
         [0024]    The flexible cable  62 , runs through a diverter sheave  66  that is mounted on the interior of the firewall  64 . As best seen in FIG. 7, a mounting plate  65  is provided to mount the diverter sheave  66  to the firewall  64 . The cable  62  has a brake pulling bracket  68  that attaches to the arm  17  of the brake pedal  16 . The diverter sheave  66  is basically a wheel that redirects the direction of the cable  62  so that it can pull the brake pedal  16 . A diverter sheave mounting bracket  67  attaches the diverter sheave  66  to the mounting plate  65 , which bolts to the firewall.  
         [0025]    The electrical components of the system can also be seen schematically in FIG. 1. In normal use, the electrical energy for the system is provided by the battery of the towing vehicle  12 . Those of ordinary skill in the art will understand that a separate source of electrical energy could be provided. The wiring for the system is identified by conventional colors. Obviously it will not be necessary for the wires to be of any particular color; however, the color scheme described herein is useful for identifying the wires, but may be varied in practice. The cold side of the towing vehicle&#39;s brake light switch  74  is wired to an on-off switch that can be manually moved between the on (closed) position and the off (open) position. A red wire  72  runs from the cold side of the on-off switch to a terminal  76  at the rear of the towing vehicle  12 . A standard wiring kit  78  connects the wires at the terminal of the towing vehicle with a similar terminal  80  at the front of the towed vehicle  10 .  
         [0026]    From the front of the towed vehicle  10 , the red wire  72  connects to a terminal on the second control valve  40 , which acts as the on-off valve for the braking system. Therefore, when the brakes on the towing vehicle  12  are applied, and the towing vehicle brake light switch  74  is thereby closed, the second control valve  40  is activated. The red wire  72  is also connected to the control switch  44 . The control switch  44  is normally in a closed position, and is wired to the first control valve  38 . The control switch  44  is pressure sensitive, and moves to the open position when the pressure in the air hose leading to the vacuum motor  14  drops below a specified level. The amount of pressure required to maintain the control switch  44  in the closed position can be adjusted by turning a screw that is accessible through the case of the control box  36 . Therefore, when the brakes on the towing vehicle  12  are applied and the brake light switch  74  is thereby closed, the first control valve  38  is activated, unless the pressure in the vacuum motor  14  is below a specified level. The first control valve  38  thereby acts as a regulator to only apply depress the towed vehicle brake pedal  16  a desired percentage, rather than fully depressing the pedal in most braking instances.  
         [0027]    A white wire  110  runs from the negative terminal of the towing vehicle battery  70  to the negative terminal of the towed vehicle batter  86 . The white wire  110  then runs to both the first control valve  38  and the second control valve  40 .  
         [0028]    In certain instances, such as “panic stops” or break away situations, it is desirable to fully apply the brakes of the towed vehicle  10 . The wiring of the system permits full application of the towed vehicle  10  brakes in these situations. The red wire  72  is also wired to a momentum switch  82 . The momentum switch  82  is wired to the first control valve  38 , and is normally open, but closes when it senses a fast change in momentum in the towed vehicle  10 . The momentum switch  82  is typically a gravity switch, such as a mercury switch, but those of ordinary skill in the art will be aware of other acceptable alternatives. Therefore, in instances where the towed vehicle  10  is slowing so rapidly that the momentum switch  82  closes, the momentum switch  82  overrides the control switch  44 , and activates the first control valve  38  even if the control switch  44  is open, so that the full vacuum created by the vacuum source  18  is applied to the vacuum motor  14 .  
         [0029]    The wiring is also structured to apply the towed vehicle brakes in a break away situation where the towed vehicle  10  has become accidentally unhitched from the towing vehicle  12  during the towing process. An orange wire  84  connects the positive terminal of battery  86  of the towed vehicle  10  to a break away switch  88  that is held open by a pin  90  connected to a break away cable  92  that attaches to the rear of the towing vehicle  12 . These break away switches are common, and several designs will work effectively with the present system. The cold side of the break away switch  88  is wired by a black wire  96  to the first control valve  38 . The cold side of the break away switch  88  is also wired to the second control valve  40 , across a diode  94 . The diode  90  prevents back current to the first control valve  38  when the break away switch  88 , momentum switch  82 , and control switch  44  are all open and the towing vehicle brakes are being applied.  
         [0030]    The wiring of the system also allows for monitoring and testing of the status of the system in several regards. A test switch  98  can be wired to the battery  70 , or other power source, on the towing vehicle  12 . This test switch  98  normally is open, but can be closed to simulate the application of brakes in the towing vehicle  12 . The cold side of the test switch  98  is wired to the red wire  72  to bypass the on-off switch  74 . A power indicator light  100  is wired on one terminal to the cold sides of both the on-off switch  74  and test switch  98 , and on the other terminal to ground, or the negative terminal of the towing vehicle battery  70 . Therefore, when braking system in the towed vehicle  10  is receiving power is going from the towed vehicle  12 , the power indicator light  100  is lit.  
         [0031]    A towed vehicle brake status indicator light  102  is also provided in the towing vehicle  12 . A blue wire  104  connects the towed vehicle brake status indicator light with the cold side of the brake light switch  106  of the towed vehicle  10 . Therefore, anytime the brakes of the towed vehicle  10  are applied, such that the brake lights of the towed vehicle are activated, the towed vehicle brake status indicator light  102  is also lit. Typically, both the towed vehicle brake status indicator light  102  and the power indicator light  100  are mounted in the cabin of the towing vehicle  12 , in plain sight of the driver of the towing vehicle  12  so that the driver can monitor the status of the system.  
         [0032]    A break away test switch  108  is also provided to permit testing of the break away function. The break away test switch  108  is typically mounted in the cabin of the towed vehicle  10 , but may also be mounted under the hood of the towed vehicle  10 . The cold side of the break away test switch  108  is wired to the black wire  96 , while the hot side of the break away test switch  108  is wired to the orange wire  84 . The break away test switch  108  is normally in an open position, but can be manually adjusted to a closed position to test whether the break away function is operational. If the system is operational, the brake pedal  16  of the towed vehicle  10  should be fully applied when the test switch  108  is moved to a closed testing position.  
         [0033]    Those of ordinary skill in the art will be aware of various options for mounting the components described above on the towed  10  and towing  12  vehicles. A preferred manner of mounting the components is described below. The first step is to disconnect the battery  86  of the towed vehicle, so that no accidental electrocution will occur. The control box  36 , vacuum motor  14 , and vacuum reservoir  34  should be mounted under the hood of the towed vehicle. It is important that these items be mounted in locations such that they are not resting against any hot engine components and that they be located so that they can be adjusted. These components may be preferably mounted with self-tapping screws.  
         [0034]    If the momentum switch  82  is a gravity switch, such as a mercury switch, it must be mounted in the proper orientation. Typically, this will mean that the momentum switch will be mounted on one of the outer walls of the engine compartment of the towed vehicle such that the wire outlet end of the momentum switch  82  points towards the front of the towed vehicle  10 . The momentum switch must be at the proper angle with respect to level in order to function properly. To help accomplish this, a tool  112  is provided that has a two opposing surfaces offset from each other by the proper angle of orientation for the momentum switch  82 . To properly orient the momentum switch  82 , the tool  112  is placed with one of the surfaces on the momentum switch  82 . A level  116  can then be placed on the opposing surface to verify that the momentum switch  82  is mounted at the proper angle. Preferably, the side of the tool  112  placed on the momentum switch  82  will have a cut-out area  114  to allow for the mounting bracket  83  of the momentum switch  82 .  
         [0035]    A standard mounting bracket should be mounted to the front of the towed vehicle  10  to permit quick connection of the wires and the vacuum line  20 . This mounting bracket should include the quick coupling  22  as well as a wire receptacle to act as the  80 . Additionally, the break away switch  88  should be mounted to the front of the towed vehicle  10 . This is a standard arrangement for which those of ordinary skill in the art will be aware of numerous alternatives.  
         [0036]    The diverter sheave  66  needs to be mounted to cabin side of the firewall  64 . This is accomplished by pulling back the carpet on the floor pan of the driver&#39;s side of the towed vehicle  10 . The battery  86  should be temporarily reconnected for this step, because it will be necessary to start the towed vehicle  10  and apply its brakes. The mounting plate  65  should be mounted directly underneath the brake pedal  16 . The mounting plate  65  can then be mounted the floor pan, and the carpet reapplied over the top of the mounting plate  65 . A small hold should be trimmed in the carpet to permit an allen bolt provided at the center of the mounting plate  65  to extend upwardly through the carpet. The diverter sheave mounting bracket  67  is then mounted to this allen bolt to connect the diverter sheave mounting bracket to the mounting plate  65 . The brake cable  62  should be fed through the firewall  64  of the towed vehicle  10 . This can usually be accomplished through the auxiliary wire boot provided in most vehicles. The cable  62  should be threaded through the diverter sheave  66 , and attached to the brake pulling bracket  68 . The brake pulling bracket  68  should be attached to the brake pedal arm  17 , as close as possible to the brake pedal  16 . The opposite end of the cable  62 , should then be connected with the rod  60  of the vacuum motor  14 .  
         [0037]    The break away test switch  108  should be mounted on the interior of the towed vehicle  10 , commonly under the dash. The orange wire  84 , and the black wire  96  should be threaded through the rubber auxiliary wire boot, and connected to the break away test switch.  
         [0038]    The blue wire  104  should be connected to the cold side of the towed vehicle brake light switch  106 . A brake light relay may be used for this purpose. Those of ordinary skill in the art will be well aware of different manners for attaching the blue wire  104  to the cold side of the towed vehicle brake light switch.  
         [0039]    The towed vehicle battery  86  can be reconnected, and the break away system can be tested and calibrated. To calibrate the system, a level area with no traffic, preferably a large vacant parking lot, should be used. The towed vehicle  10  should be started and driven, free from the towing vehicle  12 , at a speed of approximately twenty miles per hour. The test switch is then moved to the activated or closed position. The system should depress the towed vehicles brakes, and bring the vehicle to a stop within forty feet. If the vehicle travels more than forty feet before stopping, the effective length of cable  62  needs to be shortened slightly. If the vehicle stops more quickly than forty feet it will be necessary to lengthen the effective length of cable  62 . Those of ordinary skill in the art will be aware of various methods of adjusting the effective length of the cable  62 .  
         [0040]    For gas powered towing vehicles  12  the preferred vacuum is the intake manifold of the engine. For gas powered vehicles, a vacuum hose  20 a should be connected directly to an intake vacuum port on the towing vehicle&#39;s engine. For diesel powered towing vehicles  12 , an auxiliary vacuum pump should be installed. Those of ordinary skill in the art will be well aware of various types and models of vacuum pumps that are suitable. It has been found that a vacuum pump that will create a vacuum of approximately twenty-one inches of mercury should be sufficient.  
         [0041]    Preferably, the on-off switch  74 , the test switch  98 , and the indicator lights  100 ,  102  are mounted together on a power switch panel that can be mounted to the interior of the towing vehicle  12 . This should be mounted in a location that is easily visible and accessible to a driver of the towing vehicle.  
         [0042]    The on-off switch should be wired to the cold side of the brake light switch  74  of the towing vehicle. Those of ordinary skill in the art will be familiar with how to do this. One such manner is to find the cold side of the brake light switch  74  by using a wire tester and find the wire that lights the tester only when the brake pedal is depressed. The wire can then be cut, and both cut ends of the wire stripped, and then the stripped ends of the cut wire can be connected to the on-off switch using butt connectors.  
         [0043]    The indicator lights  100  and  102 , and the test switch  98  are then wired according to the structure described above so that the system should be fully operational. When connecting the test switch  98 , it is important to be sure that the wire to which it is connected has power only when the ignition key is turned on, and does not have power when the ignition key is turned off. This is typically available through an auxiliary power source outlet in the fuse panel of the towing vehicle  12 .  
         [0044]    To operate the system, it is necessary to hitch the towed vehicle  10  to the towing vehicle  12 , typically with a tow bar. An air hose  20   b  is then used to connect the quick connect couplings  22   a  and  22   b . A wiring kit  78  is used to join all of the electrical connections between the two vehicles. The break away cable  92  should be attached in place between the break away switch  88  at the front of the towed vehicle  10  and the rear of the towing vehicle  12 . The brake pulling bracket  68  should be attached to the brake pedal arm  17 . The towing vehicle  12  should be started, and the on-off switch  74  adjusted to the on position. It may be desirable to test the system by adjusting the break away test switch  108  to the on position to make sure the brake pedal  16  is depressed. Similarly, it make be a good idea to test the system by adjusting the test switch  98  to the on position to see if the brakes on the towed vehicle  10  get applied.  
         [0045]    To determine whether the control switch  44  is set to the right pressure, a few stops should be made while pulling the towed vehicle  10  behind the towing vehicle  12  with the system engaged. If the brakes of the towed vehicle  10  are cold, then the pressure setting for the control switch  44  may be set too high, such that the switch opens too soon before the vacuum reaches a high enough level to apply the brake pedal  16  as strongly as desired. Alternatively, if the brakes of the towed vehicle  10  are hotter than the brakes of the towing vehicle  12 , then the pressure may be set too low, and the brake pedal  16  of the towed vehicle  10  is being pulled too far during normal stopping. Appropriate adjustment should be made.  
         [0046]    When the brakes of the towing vehicle  12  are applied, that closes the brake light switch  74  on the towing vehicle  12 . Power is thereby control valves  40  and  38 , and they are activated to apply vacuum from the vacuum source  18  to the vacuum motor  14 . As the vacuum is applied to the vacuum motor, the vacuum motor  14  pulls cable  62 , which depresses the brake pedal  16 . When vacuum builds to a sufficient level, the control switch opens, which deactivates the first control valve  38  and breaks the pneumatic connection between the vacuum motor  14  and the vacuum source  18 . However, the vacuum that existed on the vacuum motor before the deactivation of the first control valve  38  remains constant because the exhaust port on the first control valve  38  is closed. When the brakes on the towing vehicle  12  are released, the brake light switch  74  on the towing vehicle  12  opens, and the second control valve  40  is also deactivated. The exhaust port on the second control valve goes to atmosphere, so the vacuum motor releases its pull on the brake pedal  16 , and the brakes on the towed vehicle  10  are no longer applied. In this manner, the brakes on the towed vehicle  10  are applied and released almost instantaneously with the brakes of the towing vehicle  12 . However, the brakes on the towed vehicle  10  are only partially applied to avoid unduly straining the brakes on the towed vehicle or causing skidding or pulling by the towed vehicle  10 .  
         [0047]    In the panic mode, when the momentum switch  82  closes, the both control valves  38 ,  40  are maintained in the activated position, regardless of whether the control switch  44  is open. In this manner, the regulated partial pull of the brake pedal that occurs in a typical application of the brakes is bypassed so that the full braking force can be applied.  
         [0048]    The panic mode of stopping the vehicle should not occur too early or too late. If the panic mode is engaged too early it can result in skidding of the towed vehicle. If the panic mode is engaged too late, it is not effective in helping to quickly bring both the towing vehicle  12  and towed vehicle  10  to a quick stop. To test for this, the towing vehicle  12  should fully apply its brakes while pulling the towed vehicle  10 . If the towed vehicle  10  lock or slide during this process, then the panic mode is being engage too early and the angle of the momentum switch  82  should be adjusted by raising the front end of the momentum switch  82 . If, on the other hand, the towing vehicle  12  and towed vehicle  10  are brought to a complete stop before the system goes into full braking, then the momentum switch should be adjusted lowering the front end of the momentum switch  82  so that it will be triggered more easily.  
         [0049]    In a break away situation, the breakaway cable  92  will remain attached to the towing vehicle  12 , and the pin  90  will be removed from the break away switch  88 . The break away switch  88  will therefore close, which will complete a circuit that activates both of the control valves  34 ,  40 . The vacuum stored in the vacuum reservoir  34  will activate the vacuum motor  14  and thereby depress the brake pedal  16 . The check valve  30  prevents back flow of air into the system. The reservoir  34 should hold the brake pedal  16  depressed for at least fifteen minutes following a break away.  
         [0050]    Therefore, what has been described is a system that provides supplemental braking power to a towed vehicle by utilizing the brake booster  26  of the towed vehicle  10 . The brakes of the towed vehicle  10  will only partially be deployed in standard braking situations. This permits greater control of the towed vehicle  10 , avoids unnecessary wear on the brake parts as compared to systems that fully employ the towed vehicles brakes  10  in every situation. Furthermore, the system provides the advantage of fully applying the towed vehicle  10  brakes in situations where that is important, such as a break away or a panic stop. The use of a vacuum motor having a flexible membrane provides a robust and effect means of applying the towed vehicle&#39;s brakes.