Patent Publication Number: US-2005134110-A1

Title: Brake assembly with brake response system

Description:
This application is a continuation-in-part of U.S. application Ser. No. 10/742,537, filed Dec. 19, 2003, the contents of which are hereby incorporated by reference. 
    
    
      The present invention is directed to a brake assembly, and more particularly, to a brake assembly having a brake response system which allows additional flow of brake fluid to a brake caliper.  
     BACKGROUND  
      In most existing analog brake systems, when the driver presses on the brake pedal brake fluid is forced under pressure from a master cylinder to the caliper to cause the caliper/brake pad of a wheel brake system to move against the rotor of the wheel. The frictional engagement between the rotor and the brake pad brakes the system and causes the associated wheel to decelerate in a well-known manner.  
      In antilock brake systems (“ABS”), the brake system or assembly includes an apply valve to control the flow of fluid therethrough during application of the brakes in an ABS event. The apply valve has an opening or orifice through which the fluid flows. The orifice has a defined or fixed size which is relatively small to allow controlled adjustments of the pressure in the associated caliper during an ABS or other controlled braking event. However, the limited orifice size of the apply valve may reduce the responsiveness of the brake system due to the limited flow volume which can flow through the restricted orifice. Accordingly, there is a need for an ABS brake system which includes a relatively large orifice to allow high volume flow, while still providing good control during an ABS or controlled braking event.  
      When using a brake system with a traction control system (“TCS”) or electronic stability control (“ESC”), it is often desired to route the brake fluid from the master cylinder and/or master cylinder reservoir to the inlet of the pump to allow pressurization and application of the brakes through operation of the pump. However, existing master cylinder prime valves and brake lines (which allow fluid to flow from the master cylinder to the inlet of the pump) may have limited size and flow capabilities (i.e., a limited cross sectional area).  
      In some situations, particularly on larger vehicles, it may be desired to provide a high volume of brake fluid to the inlet of the pump to achieve good brake pressure response times for either TCS or ESC systems. In particular, in cold weather conditions, the viscosity of the brake fluid may be relatively high, in which case it may prove especially useful to incorporate a relatively high volume flow of brake fluid into the system. Accordingly, there is a need for such a high volume flow system that can deliver fluid from the master cylinder and/or master cylinder reservoir to the pump inlet.  
      When utilizing a TCS, ESC, and/or ABS system, it may be desired to generate relatively high pressures by the pump or pump assembly in order to ensure proper operation of the TCS, ESC, and/or ABS system. It is also desired to provide for a stable pump arrangement which provides a uniform inlet vacuum and outlet pressure source. Accordingly, there is a need for a brake system having a pump arrangement which can generate relatively high pressures and operate in an efficient manner.  
     SUMMARY  
      In one embodiment the present invention is a brake system including a brake response valve and a brake response conduit in fluid communication with a caliper to allow the flow of fluid from the master cylinder to the caliper, while bypassing the apply valve of the ABS brake system. In particular, in one embodiment the invention is a braking system including a master cylinder, a brake subsystem for applying pressure to a brake rotor, and an apply conduit. The apply conduit is configured to selectively allow the flow of fluid therethrough from the master cylinder to the brake subsystem to thereby cause the brake subsystem to apply or increase pressure to the brake rotor. The braking system further includes a bypass conduit configured to selectively allow the flow of fluid therethrough from the master cylinder to the brake subsystem to thereby cause the brake subsystem to apply pressure to the brake rotor.  
      In another embodiment, the present invention is a brake system configured to deliver fluid from the master cylinder to an inlet of the pump. In particular, in one embodiment the present invention is a brake system which includes a reservoir prime valve and reservoir conduit in fluid communication with the inlet of the pump and the master cylinder and/or master cylinder reservoir. The reservoir prime valve and reservoir conduit allow brake fluid to flow directly from the master cylinder and/or master cylinder reservoir to the pump to allow improved operation during TCS and ESC pressure build cycles.  
      In particular, in one embodiment the invention is a braking system including a master cylinder having a reservoir, a brake subsystem for applying pressure to a brake rotor, and an apply conduit. The apply conduit is configured to selectively to allow the flow of fluid therethrough from the master cylinder to the brake subsystem to thereby cause the brake subsystem to apply or increase pressure to the brake rotor. The system includes a release conduit configured to selectively to allow the flow of fluid therethrough and away from the brake subsystem to thereby cause the brake subsystem to reduce any pressure to the brake rotor. The system further includes a pump having a pump inlet and a pump outlet, wherein the release conduit is in fluid communication with the pump inlet. The system includes a reservoir conduit configured to selectively allow the flow of fluid therethrough from the master cylinder to the pump inlet.  
      In another embodiment, the present invention is a brake system including various pump configurations for improved pumping operations. In particular, in one embodiment the invention is a braking system for a vehicle including a master cylinder, a fluid line assembly in fluid communication with master cylinder and at least one brake disposed at a wheel of the vehicle, and a pumping unit in fluid communication with the fluid line assembly. The pumping unit includes toro fluid pumping elements that are arranged to be about 180 degrees out of phase with each other during operation.  
      Other objects and advantages of the present invention will be apparent from the following description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic representation of a baseline ABS brake system with a front/rear split;  
       FIG. 1A  is a schematic representation of the valve layout of the system of  FIG. 1 ;  
       FIG. 1B  is a simplified schematic representation of the front brake circuit of the system of  FIG. 1 ;  
       FIG. 2  is a schematic representation of the brake system of claim  1 , modified to include a brake response valve;  
       FIG. 2   a  is a schematic representation of the valve layout of the system of  FIG. 2 ;  
       FIG. 2   b  is a simplified schematic representation of the front brake circuit of  FIG. 2 ;  
       FIG. 3  is a schematic representation of the brake system of  FIG. 1 , modified to include two brake response valves;  
       FIG. 4  is a schematic representation of a baseline 4-channel ESC brake system with a front/rear split;  
       FIG. 4   a  is a schematic representation of the valve layout of the system of  FIG. 4 ;  
       FIG. 5  is a schematic representation of the brake system of claim  4 , modified to include a brake response valve;  
       FIG. 5   a  is a schematic representation of the valve layout of the system of  FIG. 5 ;  
       FIG. 6  is a schematic representation of a hybrid braking system with the addition of a brake response valve and reservoir prime valve;  
       FIG. 7  is a schematic representation of a brake system with an ESC system having a brake response valve, reservoir prime valve and two 180° pump elements;  
       FIG. 7   a  is a simplified schematic representation of the system of  FIG. 7 ;  
       FIG. 7   b  is a schematic representation of the pumping arrangement of the system of  FIG. 7 ;  
       FIG. 8  is a schematic representation of the brake system of  FIG. 7 , utilizing two 90° pump elements;  
       FIG. 8   a  is a simplified schematic representation of the system of  FIG. 8 ;  
       FIG. 9  is a schematic representation of the brake system of  FIG. 7 , with a “3+1” pump configuration;  
       FIG. 9   a  is a simplified schematic representation of the system of  FIG. 9 ;  
       FIG. 10  is a schematic representation of the brake system of  FIG. 3  utilizing two 180° pump elements;  
       FIG. 11  is a schematic illustration of a braking system according to one embodiment of the invention;  
       FIG. 12  is a graph showing fluid pressure over the operating cycles of pumps according to one embodiment of the invention; and  
       FIG. 13  is a schematic illustration of a braking system according to another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
      As shown in  FIG. 1 , a basic or baseline ABS brake system  10  includes a master cylinder  12  having a master cylinder reservoir  14  storing excess brake fluid therein. The master cylinder  12  includes a pair of pistons (not shown) located therein that are isolated from each other inside the master cylinder  12 . Each piston is coupled by way of mechanical or fluid means to a rod  16  which protrudes outwardly from a brake booster  12   a , which in turn activates or controls pressure in the master cylinder  12 . The rod  16  is coupled to a brake pedal (not shown). The position of the rod  16  and brake pedal is monitored by brake switch  76  operatively coupled to the rod  16  by a linkage  18 . The master cylinder  12 , pistons, brake booster  12   a , rod  16 , and brake pedal are configured so that when a driver depresses the brake pedal, the rod  16  and pistons are moved (i.e., to the left in the drawing of  FIG. 1 ), thereby pressurizing the fluid in the master cylinder  12 .  
      The master cylinder  12  includes a pair of outlet ports  20 ,  22  with a primary main brake line  24  coupled to and extending from the port  20 , and a secondary main brake line  26  coupled to and extending from the other port  22 . Each of the primary brake line  24  and secondary main brake line  26  (also termed fluid line assemblies, or collectively a fluid line assembly), as well as other lines or conduits described herein, may include or be defined by fluid lines, fittings, line connectors, valves and the like.  
      Turning first to the primary brake subsystem, or front brake subsystem or circuit, fluid in the primary main brake line  24  is pressurized by one of the pistons when a driver presses the brake pedal, and the primary main brake line  24  is in fluid communication with a pair of front normally open apply valves  28 .  FIG. 1  schematically illustrates the apply valves  28  in their open position which allows the flow of fluid therethrough. The apply valves  28  are actuable or movable (i.e., to the right from their position shown in  FIG. 1 ) to their closed positions to block the flow of fluid therethrough. The system includes a pair of front apply check valves  30  in parallel with the associated apply valve  28 .  
      Fluid passing through each apply valve  28  flows to the associated front wheel brake system  32  (i.e., associated with either the right front (“RF”) or left front (“LF”) wheel). Pressurized fluid thereby causes the caliper  34  of the wheel brake system  32  to move and thereby cause the brake pad or brake pads press against the rotor of the wheel  36  to cause braking of the wheel  36  in a well-known manner.  
      Each wheel brake subsystem  32  and the outlet of each apply valve  28  is also in fluid communication with an associated front normally closed release valve  38 . The release valves  38  are shown in  FIG. 1  in their closed positions, wherein the release valves  38  block the flow of fluid therethrough. Each of the release valves  38  is actuable or movable (i.e., to the right in  FIG. 1 ) to its open position to allow the flow of fluid therethrough. The system includes a pair of front release check valves  40 , each check valve  40  being in parallel with its associated release valve  38 .  
      The outlet of each release valve  38  is in fluid communication with a front or primary circuit accumulator  42  such that fluid flowing from the release valves  38  can flow to the primary circuit accumulator  42 . The primary circuit accumulator  42  and the fluid stored therein are in fluid communication with the pump, generally designated  44 . The pump  44  includes a motor  46  which reciprocally drives a pair of pistons (not shown in  FIG. 1 ), each piston being located in a cylinder or pumping chamber  48 . Each pumping chamber  48  includes an inlet check valve  50  and an outlet check valve  52  in a flow-through orientation, such that reciprocal movement of the pistons drives the fluid in the direction of arrows  54  (i.e., upwardly in  FIG. 1 ) in the well-known manner of a positive displacement pump.  
      When the pump  44  is operating, fluid exits the pumping chambers  48  and enters a damper chamber  56 . The pumped fluid then passes through an orifice  58  designed to restrict flow to create a controlled back pressure in damper chamber  56 , which in turn reduces the amplitude of pressure pulsations of the fluid being returned to primary main brake line  24 . The orifice  58  therefore reduces noise and brake pedal harshness experienced by the driver during an ABS or other controlled brake event.  
      The primary main brake line  24  provides fluid flow to the right front and left front wheel brake systems  32 . Turning now to the rear wheels or the secondary brake system, the secondary main brake line  26  is coupled to port  22  to provide fluid flow to the rear wheel brake systems  60  associated with the right rear (“RR”) and left rear (“LR”) wheels. The fluid to the secondary main brake line  26  is pressurized by a piston located in the master cylinder that is separate from the piston that pressurizes fluid in the primary main brake line  24 . In this manner, two separate, isolated hydraulic systems or brake circuits for the front and rear wheel brake systems are provided to provide a front/rear split for brake redundancy in a well-known manner. The front (primary) brake system can be considered to include all of the conduits, valves, fittings, pump portions, components etc. that are wetted by fluid flowing from port  20  and primary main brake line  24 . The rear (secondary) brake system can be considered to include all of the conduits, valves, fittings, pump portions, components, etc. that are wetted by fluid flowing from port  22  and secondary main brake line  26 . The front and rear brake systems can be considered, separately or together, as a fluid line assembly.  
      The secondary main brake line  26  is in fluid communication with a rear apply valve  62  and a rear release valve  64 , and associated check valves  66 ,  68 , which operate in a similar manner to the valves  28 ,  30 ,  38 ,  40  discussed above in the discussion of the primary (front) brake subsystem. In the illustrated embodiment, the rear wheel brake systems  60  are commonly controlled by a single apply valve  62  and a single release valve  64 , although separate valves and separate control for each of the rear wheels may be utilized if desired. The rear release valve  64  is in fluid communication with a rear or secondary circuit accumulator  70 , which provides fluid to a pumping chamber  48 . Fluid passing through the pump  44  then passes through the damper  56  and damping orifice  58  of the rear system in the same manner as the front or primary system.  
      The brake system  10  may include a plurality of sensors to monitor the status of the vehicle. In particular, the brake system  10  may include wheel speed sensors  72  associated with the front wheels, a fluid level sensor  74  for detecting fluid level in the master cylinder reservoir  14 , the brake pedal position sensor  76 , and a transmission sensor  78  which measures the speed of the output shaft of the transmission to thereby provide a measurement of the average speed of the two rear wheels. Each of the sensors  72 ,  74 ,  76 ,  78  are operatively coupled to an electronic control unit (“ECU”)  80  which can receive and/or process inputs from the various sensors  72 ,  74 ,  76 ,  78 . The ECU  80  is also coupled to each of the apply and release valves  28 ,  38 ,  62 ,  64 , as well as the pump motor  46  to control and monitor these components. The ECU  80  is shaped to be mated with a hydraulic control unit (“HCU”) (not shown) which includes hydraulic controls. Thus, when the ECU  80  and HCU are mated together, they form a hydraulic and electric control unit (“EHCU”), which includes and integrates hydraulic and electronic control elements.  
      Existing ECU units  80  may include a limited number of input ports and/or output ports such that the ECU  80  can only monitor a number of sensors that is equal to the number of input ports, or it may be limited as to the number of output ports it can operate. For example, the ECU  80  may include only eight or twelve (or various other numbers) of solenoid output ports, each output port including a solenoid coil and the associated electronic hardware necessary to operate the coil. As shown in  FIG. 1   a , for an eight-coil ECU unit, the system of  FIG. 1  utilizes only six of the eight coils or output ports  82  (i.e., for the release and apply valves), leaving two open output ports  82 ′.  
      During ABS control, the apply  28 ,  62  and release  38 ,  64  valves are operated to control the brake pressure applied to the associated rotors/wheels so that the applied pressure matches, as closely as possible, the pressure requested by the driver while regulating wheel slip to provide the maximum brake torque available for the given tire/road interface. Thus, the apply  28 ,  62  and release  38 ,  64  valves, as well as the pump  44 , operate to control braking pressure in the well-known manner of ABS control. In particular, when it appears that a wheel is approaching a full lock condition, the associated release valve  38 ,  64  is moved to its open position to reduce braking pressure to reduce wheel slip. When the wheel slip level has been sufficiently reduced and it appears that braking pressure can be increased, brake pressure is incrementally increased by quickly pulsing open the associated apply valve  28 ,  62  while the release valves  38 ,  64  remain closed. The pump  48  operates continuously during the ABS cycle to return any fluid flowing from a released brake caliper  34  back to master cylinder  12 . In the system shown in  FIG. 1 , individual ABS control is provided for each of the front wheels, and the rear wheels are commonly controlled.  
      The incremental increase in pressure (“pressure build-up”) implemented by opening the apply valves  28 ,  62  while the pump is operating and the release valves  38 ,  64  are closed should be precisely controlled. Accordingly, the flow orifice of the apply valves  28 ,  62  may be relatively small or restricted to provide for precise control during pressure buildup. However, the restricted orifice of the apply valve  28 ,  62  may limit the response time of braking during normal (non-ABS) braking.  
      The system of  FIG. 2  addresses the issues raised by the restricted orifice of the front apply valves  28  through the addition of a brake response system  88 , which includes a brake response conduit  90  and a brake response valve  92  located in the brake response conduit  90 . In particular, the brake response conduit  90  fluidly couples the master cylinder  12  (via outlet port  20  and the primary main brake line  24 ) to the wheel brake systems  32  of both the right front and left front wheels. The brake response valve  92  is located in the brake response conduit  90  to control the flow of fluid therethrough. The brake response conduit  90  includes a pair of sub-lines  90   a ,  90   b , each of which is coupled to one of the wheel brake systems  32  and includes a brake response check valve  94  located therein.  
      When a driver presses on the brake pedal during normal braking operations, fluid exiting port  20  and flowing through the primary main brake line  24  may flow through the brake response conduit  90  and brake response valve  92  directly to each of the wheel brake systems  32 . The brake response valve  92  is a normally open valve which allows brake fluid to flow therethrough during normal brake operations. The brake response valve  92  can be moved to its closed position during controlled braking events, such as ABS operation. The brake response check valves  94  allow pressure in each of the right front and left front wheel brake systems  32  to be isolated and individually controlled, for example, during ABS operation.  
      The brake response valve  92 , check valves  94  and brake response conduit  90  (and sub-lines  90   a ,  90   b ) may have a relatively large cross section area or orifice to allow for quicker response times and greater braking forces in shorter times. This can allow the braking system to be used on heavier vehicles (for example light trucks) without resizing the existing valves and components, although the brake response system  88  can also be used with cars or other vehicles. For example, the apply valve  28  may have a circular orifice having a diameter of about 0.7 mm. The brake response valve  92  may have a diameter of, for example, between about 0.85 mm and about 1.0 mm (or greater) and the brake response conduit  90  and check valves  92  may have an even larger diameter. Because the flow through an orifice is related to its cross-sectional area, in this case a 0.85 mm brake response valve  92  provides an area that is about 1½ times greater than that of the 0.7 mm of the apply valve  28 , while a 1.0 mm brake response valve  92  provides an area that is about 2 times greater than that of the 0.7 mm of the apply valve  28 .  
      During normal braking, brake fluid may flow through both the apply valve  28  and the brake response valve  92 . Thus, the brake response system  88  provides an additional flow path parallel to the flow path provided through the apply valves  28  and allows significantly increased flow of brake fluid during braking. This allows for a larger volume flow rate from the master cylinder  12  to the wheel brake systems  32 , and provides a quicker response time. Further, during controlled braking operations such as ABS, the brake response valve  92  can be closed to allow pressure to be generated, controlled and/or modulated in the primary brake system. Although the brake response system  88  is illustrated in  FIG. 2  in conjunction with a brake system having a front/rear split, the brake response valve  92  and system  88  may also be used in a brake system having a diagonal split or other arrangements. The brake response valve  92  may be an infinitely adjustable, linear valve that can regulate flow in proportion to applied current. Alternately, the brake response valve  92  may be a simpler “on/off” valve which is pulsed closed with full voltage or otherwise maintained in the fully open position.  
       FIG. 2  illustrates the brake response conduit  90  as having a pair of sub-lines  90   a ,  90   b . However, rather than having two sublines  90   a ,  90   b  which share a common source line  90  and both of which are controlled by brake response valve  92 , each of the wheel brake systems  32  may have its own, dedicated brake response conduit and brake response valve. In this case, each of the sub-lines  90   a ,  90   b  may be directly coupled to the master cylinder  12  and to the primary main brake line  24 , and each sub-line may have a controllable brake response valve located therein.  
       FIG. 2   a  illustrates the valve output coil or output port setup of an ECU  80  of the system of  FIG. 2 . As can be seen, the addition of the brake response valve  92  utilizes one of the empty output ports  82 ′ of the system of  FIG. 1   a . The systems of  FIG. 2  and  2   a  only utilize 7 output ports of the ECU and thus can be easily accommodated into many ECUs.  
       FIG. 3  illustrates the system of  FIG. 2 , with an additional brake response system  98  in the form of rear brake response valve  100  and rear brake response conduit  102  coupled to the secondary main brake line  26  and to the rear wheel brake systems  60  to provide an additional flow path to the rear wheels. Accordingly, this rear brake response system  98  provides the same advantages (in the form of increased flow of brake fluid to the rear wheel brake systems  60  during normal braking) as the brake response system  88  discussed above in the context of  FIG. 2 . Further, the system of  FIG. 3  utilizes eight controllable valves  28 ,  38 ,  92 ,  100 ,  62 ,  64 , and thus can still be easily accommodated in the ECU  80  shown in  FIGS. 1   a  and  2   a.    
       FIG. 4  illustrates a brake system  110  utilizing electronic stability control (“ESC”), also referred to as vehicle stability enhancement (“VSE”) or an electronic stability program (“ESP”), which is an electromechanical control system designed to monitor and influence wheel dynamics, and ultimately vehicle dynamics, during a vehicle state of braking, accelerating or coasting (termed ESC for the purposes of this application). ESC typically uses input from wheel speed sensors  72 , a steering wheel angle sensor  112 , a yaw rate sensor  114  and a lateral acceleration sensor  116  and optionally a longitudinal acceleration sensor  118  to determine the driver&#39;s intended heading and the vehicle&#39;s actual heading. ESC may be designed to identify the intent of a driver by measuring the steering wheel angle, brake and throttle positions and vehicle speed. The ESC typically controls the application of a brake on a single wheel, as necessary, to help a driver regain control in a skid caused by oversteering or understeering on a curve, but also can operate to provide control and vehicle guidance in various other manners.  
      In order to accommodate the ESC system, various sensors beyond the sensors of the systems of  FIGS. 1-3 , including the yaw rate sensor  114 , steering angle sensor  112 , a lateral acceleration sensor  116 , and longitudinal acceleration sensor  118  may be utilized and operatively coupled to the ECU  80 . In addition, an engine throttle control  120  and pressure sensor  122 , which monitors the pressure of the brake fluid, may be utilized and operatively coupled to the ECU  80 . Finally, in the embodiment shown in  FIG. 4 , both of the rear wheels also include wheel individual speed sensors  72 .  
      The brake system shown in  FIG. 4  includes a pair of prime valves  130 ,  132  and a pair of isolation valves  134 ,  136 . Each prime valve  130 ,  132  is coupled to the associated main brake line  24 ,  26  such that fluid can flow from the master cylinder  12  to the inlet of the pump  44  via the associated prime conduit  140 ,  142 . Each prime valve  130 ,  132  is a normally closed valve and each isolation valve  134 ,  136  is a normally open valve. Each isolation valve  134 ,  136  is placed in the associated main brake line  24 ,  26  to allow or block the flow of fluid between the master cylinder  12  and the associated apply valves  28 ,  62  and wheel brake systems  32 ,  60 .  
      The brake system  110  of  FIG. 4  further includes a pair of isolation check valves  146 , with each isolation check valve  146  being in parallel with the associated isolation valve  134 ,  136 . The system  110  also includes a pair of prime check valves  148 , with each prime check valve  148  being in parallel with the associated prime valve  130 ,  132 . The system  110  further includes a pair of external pump check valves  150 , with each external pump check valve  150  being located between one of the accumulators  42 ,  70  and the input of the associated pump chamber  48 .  
      When utilizing the ESC system in a traction control cycle, it is generally desired to apply brake pressure to an excessively spinning wheel during acceleration to thereby cause torque to transfer to the other wheel on the same axle in a well-known manner. Thus, this type of pressure build mode requires brake fluid to flow from the master cylinder  12  to the appropriate apply valves  28 ,  62  and associated wheel brake systems  32 ,  60  without any user input.  
      In order to operate in a traction control mode, for example to control the front brakes in a side-to-side torque distribution, the prime valve  130  is moved to its open position and the pump  44  is operated to pump fluid from the master cylinder  12  to the inlet of the pump chamber  48 , and then to move the fluid through one of the opened apply valves  28  to the associated wheel brake system  32 . Simultaneously, the isolation valve  134  is moved to its closed position to allow the system to pressurize since the master cylinder  12  is typically vented to atmosphere when the brakes are not applied by the driver. Thus, the isolation valve  130  and prime valve  134  operate in tandem, such that actuation or opening of the prime valve  130  normally accompanies actuation (i.e., at least a partial closing) of the isolation valve  134 .  
      In the illustrated embodiment, the isolation valve  134  is a variable or infinitely adjustable valve so that pressure and flow through the isolation valve  134  can regulated in proportion to the current supplied to its associated solenoid coil. This variable nature of the isolation valve  134  allows the back pressure in the system to be maintained at the desired level during operation of the traction control cycle. The apply  28  and release  38  valves may also be operated to ensure that the correct and desired brake pressure is applied to the associated wheel (i.e., to reduce slippage and to transfer torque in the well-known manner of TCS operation).  
      Operation of the isolation valve  136 , prime valve  132 , apply valves  62  and release valves  64  for the rear brake circuit during traction control mode is generally the same as that described above for the front brake circuit. The system of  FIG. 4  provides individual ABS control (in the form of apply valves  62 , release valves  64 , and associated components) for each of the rear wheels. The rear isolation valve  136  may also be a variable valve to provide similar control of brake pressure as described above.  
      As shown in  FIG. 4   a , the brake system of  FIG. 4  utilizes twelve controllable valves; that is, two front apply valves  28 , two front release valves  38 , two rear apply valves  62 , two rear release valves  64 , a front prime valve  130 , a rear prime valve  132 , a front isolation valve  134 , and a rear isolation valve  136 . Thus the ECU  80  requires twelve output ports  82  each including a solenoid valve coil and its associated electronic drivers to control the twelve active valves of the system  110  of  FIG. 4a . Accordingly, if it is desired to add a brake response system with a brake response valve (similar to the systems of  FIGS. 2 and 3 ) while using the same ECU unit, changes to the configuration of the system of  FIG. 4  must be implemented.  
       FIG. 5  illustrates the system  110  of  FIG. 4  modified to include a brake response system  151  (including a brake response valve  152 , a brake response conduit  154 , a pair of sub-conduits  154   a ,  154   b  and check valves  156 ) incorporated into the front brake system. This brake response system  151  allows fluid to flow from the master cylinder  12  to the brake subsystems  32  for the right front and left front wheel, much in the manner shown in  FIG. 2  and in the accompanying description.  
      The addition of the brake response system  151  to the system  110  of  FIG. 4 , without any additional changes, would increase the total number of the valves of the system of  FIG. 4  to thirteen valves. However, the ECU  80  utilized with the system of  FIG. 4  may include only twelve solenoid valve coil output ports, and therefore some redesign may be required in order to accommodate the additional controllable valve in the form of the brake response valve  152 .  
      In order to accommodate the brake response valve  152 , the prime valve  132  and isolation valve  136  for the rear brake system of  FIG. 4  may be combined into a single two-position three-way valve  160 . The three-way valve  160  is biased into the position shown in  FIG. 5 . Thus, during normal braking operations, the secondary main brake line  26  and port  22  of the master cylinder  12  are in fluid communication with both rear apply valves  62  and the associated wheel brake systems  60  so that the brake system  110  may operate in its normal manner. In this configuration the inlet of the pump  44  is not in direct fluid communication with the master cylinder  12 .  
      In contrast, when rear TCS braking action is required, the three-way valve  160  shifts to the right from its position shown in  FIG. 5 , such that the secondary main brake line  26  is in fluid communication with pump inlet line  164  to provide a direct line of fluid communication from the master cylinder  12  to the inlet of the pump  44 . Furthermore, when the three-way valve  160  is in its activated position, the secondary main brake line  26  and master cylinder  12  are isolated from the apply valves  62  and associated wheel brake systems  60 , which allows pressure to be generated in the rear brake system during TCS operations.  
      The system of  FIG. 5  includes a check valve  166  and a mechanical blow-off valve  168  in parallel with the three-way valve  160 . The blow-off valve  168  allows excess pump pressure to escape across the three-way valve  160  when sufficient pressure is generated by pump  48 . The blow-off valve  168  operates as a release or a check valve and sets a constant back pressure as opposed to the variable back pressure which can be provided by the isolation valves  136  or  134 . The system of  FIG. 5  also includes a check conduit  172  with a check valve  170  located therein. The check conduit  172  extends from the secondary main brake line  26  to the outlet of the three-way valve  160  and the pump inlet line  164  and allows for the pump inlet circuit to be evacuated of any entrapped air during assembly plant installation by use of industry standard evacuate-and-fill procedures.  
      Accordingly, the three-way valve  160  of  FIG. 5  can serve as a substitute of the separate rear isolation valve  136  and rear prime valve  132  of  FIG. 4 . However, the three-way valve  160  provides lesser control as compared to separate isolation  136  and prime  132  valves. For example, as noted above, the isolation valve  136  of  FIG. 4  may be a variable valve to provide greater control of the backpressure in the system that can result in lower noise levels and higher operating efficiencies. In contrast, the three-way valve  160  does not provide variable pressure control. However, incorporation of a fixed blow-off pressure is a cost-effective means of providing system pressure control and may be perfectly adequate, particularly for rear wheel pressure control that may have less stringent control requirements.  
       FIG. 5   a  is a schematic representation of the valve layout of the system of  FIG. 5 . As can be seen, the three-way valve  160  (labelled “TCS” in  FIG. 5   a ) provides control to both the left rear and right rear wheel brake systems, and allows control utilizing only twelve output ports  82  of the ECU  80 .  
       FIG. 6  is a schematic representation of a hybrid brake system  200 . In particular, in a system of  FIG. 6 , the rear wheel brake systems are completely electronically controlled for example, such as in a brake-by-wire system. In this manner, the rear wheel brake subsystems incorporate electromechanical devices to actuate the rear wheel brakes. Thus, each rear brake subsystem  60  includes a motor  202  which can be actuated by its associated remote ECU  204  to cause to its associated caliper  34  to engage the associated rotor  36  and cause braking. Each rear wheel ECU  204  is operatively coupled to the main ECU  80 , and is also coupled to a safety switch  206  that is connected to ground.  
      The front brake system is a hydraulically controlled system. The system of  FIG. 6  includes a brake response conduit  154  and sublines  154   a ,  154   b  connected to the right front and rear front wheel brake subsystems  32 , a brake response valve  152  located in the brake response conduit  154 , and brake response check valves  156  located in the sublines  154   a ,  154   b.    
      Because the rear wheel brake subsystems  60  are electronically controlled, the pump  44  does not supply any brake fluid to the rear brake circuit. Accordingly, in the system of  FIG. 6 , the outlet of both of the pumping chambers  48  are commonly fed to a single damper  56 , which then feeds the brake fluid into the front brake system or brake circuit for the front wheels. Accordingly, the pump  44  provides two pressure pulses to the front system per each motor revolution which results in smoother operation. The pump  44  may also be easily sized for additional flow, thereby providing quicker response times.  
      When utilizing the traction control mode, it is desired to provide a high volume rate of flow to the inlet of the pump  44  to thereby provide brake fluid to the brake subsystems as quickly as possible. In many existing brake systems, for example, the system shown in  FIG. 4 , fluid is provided to the inlet of the pump via the prime conduit  140  and prime valve  130 . However, due to limitations of the system such as internal restrictions in the master cylinder  12 , brake pipe sizing limitations, the orifice and/or surface area of the prime valve  130  or prime conduit  140  may be relatively low, which limits the flow of brake fluid to the inlet of the pump  44 . In particular, during cold weather applications, the viscosity of brake fluid increases significantly, which can adversely affect the flow rate of brake fluid to the pump inlet via the prime valve  130 .  
      Accordingly, the system of  FIG. 6  addresses this issue by the presence of a reservoir conduit  220  extending from the reservoir  14  of the master cylinder  12  to the inlets for both pumping chambers  48  (which inlets are collectively termed a pump inlet  206 ). The reservoir conduit  220  includes a reservoir prime valve  222  located therein to selectively control the flow of fluid from the reservoir  14  to the inlet of the pump  44 . The diameter (or cross sectional area) of the reservoir conduit  220  and the reservoir prime valve  222  may be relatively large to allow high volume flow of brake fluid from the reservoir  14  to the pump  44  since the reservoir always remains at atmospheric pressure levels.  
      The reservoir prime valve  222  may be a two-position valve that is biased into its closed position. During TCS or other controlled brake pressure build operations, the reservoir prime valve  222  may be moved to its open position to allow the flow of fluid from the reservoir  14  to the pump inlet  206 . Furthermore, because the reservoir conduit  220  is in fluid communication with the reservoir  14 , pumping efficiency may be increased because the pump  44  does not have to pull against any pressure in the master cylinder  12  due to the fact that the reservoir  14  is vented to atmosphere. However, if desired the reservoir conduit  220  may instead, or in addition, be coupled to the master cylinder  12 .  
      The reservoir prime valve  222  may be a poppet valve that is biased into its closed position. In other words, the valve  222  as a whole may be a poppet valve and may be operate in the same manner as, for example, valve  152 , even though the schematic representation of valve  222  is slightly different from that of valve  152 . The poppet valve  222  may be activated and opened to allow an additional flow of fluid during start up of the pump or during priming when the highest flow rates are desirable. However, as the system nears its target pressure, the reservoir prime valve  222  may be de-activated and closed to avoid system over-pressurization.  
       FIG. 7  represents somewhat of a combination of the system of  FIG. 5  and  FIG. 6 . In particular, the system  250  of  FIG. 7  includes the brake response system  151  for the front wheels and the three-way valve  160  for the rear wheels similar to the system of  FIG. 5 . Further, the system  250  of  FIG. 7  includes the reservoir conduit  220  and reservoir prime valve  222  of  FIG. 6  to provide rapid pumping response to the primary (front) brakes. In particular, in the system of  FIG. 7  the reservoir prime valve  222  of  FIG. 6  has replaced the master cylinder prime valve  130  of the system of  FIG. 5 . The master cylinder prime valve  130  may be omitted in the system of  FIG. 7  order to retain the overall number of valves in the system at twelve in order to accommodate the twelve solenoid coil output ports in the ECU  80 .  
      The reservoir prime valve  222  may be used instead of the master cylinder prime valve  130 . However, because the reservoir prime valve  222  is coupled to the reservoir  14  and the reservoir  14  is vented to the atmosphere, changes in the control of pressure in the system are required to accommodate the change from a master cylinder prime valve  130  ( FIG. 5 ) to a reservoir prime valve  222  ( FIG. 7 ). Further, the system  250  of  FIG. 7  includes check valves  260  that communicate with the outlet of the associated accumulator  42 ,  70 . The system  250  of  FIG. 7  also includes a check valve  264  in communication with the outlet of the front damper  56  and the reservoir conduit  220 , which allows for the pump inlet circuit to be evacuated of any entrapped air during assembly plant installation by use of industry standard evacuate-and-fill procedures.  
      The system of  FIG. 7  further includes a pump arrangement  44  configured to provide additional pumping power. In particular, the pump system  44  shown in  FIG. 7  has four cylinders (pumping chambers)  48   a ,  48   b ,  48   c ,  48   d , with a piston  51   a ,  51   b ,  51   c ,  51   d  reciprocally disposed inside its associated cylinder  48   a ,  48   b ,  48   c ,  48   d . For example,  FIG. 7   b  illustrates a pair of pump units  280   a ,  280   b , with each pump unit  280  including two opposed pistons  51 , each piston  51  being slidably received in an associated cylinder  48 . Each piston  51  is coupled to a driveshaft eccentric  282  by an associated retainer  284 . During operation, the eccentric  282  is rotated in the direction of arrow A to cause the pistons  51  to reciprocate in the associated cylinders  48 . The pistons  51  of a given pump unit  280   a ,  280   b  are offset such that, for example, when piston  51   a  (or  51   c ) is in its fully extended position, the opposing piston  51   b  (or  51   d ) is in its fully retracted position. Thus the pistons  51  of a single pump unit  280  are offset one hundred and eighty degrees.  
      Each pump unit  280  may be commonly coupled to the same driveshaft eccentric  282 . In the embodiment shown in  FIG. 7   b , the two pump units  280  are one hundred and eighty degrees out of phase such that, for example, when piston  51   a  is in its extended position piston  51   c  is in its retracted position, and when piston  51   a  is in its retracted position piston  51   c  is in its extended position. However, if desired the pump units  280  may be in phase such that the piston  51   a  and piston  51   c  move together. The pump units  280  may also out of phase by varying amounts besides one hundred eighty degrees, including ninety degrees, two hundred and seventy degrees, or other varying degrees. In the arrangement shown in  FIG. 7   b  the pump units  280  are one hundred eighty degrees out of phase which provides mechanical balance and a relatively constant inlet vacuum and outlet pressure source.  FIG. 7a  schematically illustrates the system of  FIG. 7 .  
      The system of  FIGS. 7, 7   a  and  7   b  includes four pistons  51  and cylinders  48 . In the illustrated embodiment, pumping cylinders  48   c  and  48   d  are in fluid communication with the rear wheel brake system or circuit to provide pressurized brake fluid thereto, and cylinders  48   a  and  48   b  are in fluid communication with the front wheel brake system or circuit. Because the pistons  51  of a single pump unit  280  are one hundred eighty degrees out of phase, a relatively constant pressure source is provided. If desired, cylinders  48   a  and  48   c  may be coupled to one of the brake circuits, and cylinders  51   b  and  51   d  may be coupled to the other brake circuit. Because pistons  51   a  and  51   c  are one hundred eighty degrees out of phase, as are pistons  51   b  and  51   d , a relatively constant pressure source can also be supplied in this manner. Thus in the pumping arrangement shown in  FIG. 7 , the pump units  280  are one hundred eighty degrees out of phase, and the pistons  51  that are coupled to a single line are also one hundred and eighty degrees out of phase. However, the system of  FIG. 7  may be configured such that the pumping units are in phase or out of phase by varying degrees.  
       FIG. 8  illustrates the system of  FIG. 7 , with the exception that the arrangement of the pumping elements/pistons has been modified. In particular, pumping elements/pistons and cylinder  48   a / 51   a  and  48   d / 51   d  provide pressure to the rear brake subsystem, and pumping elements/pistons and cylinders  48   c / 51   c  and  48   b / 51   b  provide pressure to the front brake subsystem.  
      Furthermore, in the arrangement of  FIG. 8  the pumping units  280   a ,  280   b  may be ninety degrees out of phase, rather than the one hundred eighty degrees out of phase shown in  FIG. 7   b . In other words, when the pistons  51   a ,  51   b  of one pumping unit  280   a  are fully extended/retracted, the pistons  51   c ,  51   d  of the other pumping unit  280   b  are located the midpoint of their stroke. This ninety degree out-of-phase arrangement provides for more efficient mechanical performance because the pistons  51  are not pulling/pushing against the maximum torque. However, the system of  FIG. 8  may be configured such that the pumping units  280   a ,  280   b  are in phase or out of phase by varying degrees.  
       FIG. 9  illustrates yet another arrangement of the pumping elements. In the system of  FIG. 9 , three of the pistons or pumping elements  48   a ,  48   b ,  48   c  provide pressure to the front brake subsystem, and only a single piston or pumping element  48   d  provides pressure to the rear brake subsystem. This “3+1” arrangement of pumping elements maximizes performance of the front brake subsystem, which typically carries a great proportion of the braking load. In the system of  FIG. 9  the pumping units  280  may be in phase or out of phase by varying degrees.  
       FIG. 10  illustrates a brake system utilizing front  92  and rear  100  brake response valves, similar to the system of  FIG. 3 . However, the system of  FIG. 10  includes four pumping elements or pistons  48   a ,  48   b ,  48   c ,  48   d  arranged in a one hundred eighty degree offset, similar to the system of  FIG. 7 . Of course, any of the arrangements of pumping elements, arrangement of brake response systems and reservoir prime valves may be utilized in nearly any combination disclosed herein.  
       FIGS. 7-10  illustrate the pumping units  280  in and out of phase by varying degrees, and the pistons  51  can be arranged to provide pressure to varying brake circuits. Thus it can be seen that the phase of the pumping units  280  and the connections of the pistons  51  to the brake circuits can be varied in a wide variety of manners to achieve the desired performance with known trade-offs.  
      Referring now to  FIG. 11 , the invention provides a braking system  410  for a vehicle. The system  410  includes a master cylinder assembly including master cylinder  412  in communication with a reservoir  414 . A first fluid path or line  416  extends between a primary port  458  of the master cylinder  412  and one or more brakes  418 ,  420  disposed at respective wheels  422 ,  424 . The line  416  can be defined by fluid lines, fittings, line connectors and valves. In the exemplary embodiment of the invention, the first fluid line  416  is part of a master cylinder primary circuit. A master cylinder to wheel circuit isolation valve and a plurality of wheel brake apply valves are shown positioned along the fluid line  416 . The braking system  410  is shown as a Front/Rear/Rear system wherein both front brakes are controlled by a single circuit and rear, electrically-actuated brakes  446 ,  448 . However, the invention is not limited to the exemplary embodiment shown but can be incorporated with any configuration of braking system including a pre-charge.  
      A second fluid path or line  426  extends between a first position  428  along the first fluid line  416  and a second position  430  along the first fluid line  416 . In the exemplary embodiment of the invention, the second fluid line  426  includes line portions  460 ,  462 ,  464 ,  466 . A master cylinder to pump prime valve  444  is disposed between line portions  460  and  462 . Line portions  462  and  464  are fluidly connected to one another at point  440 . Pumps  432  and  432   a  are disposed in parallel to one another between line portions  464  and  466 . A pump damper chamber  436  and an orifice  454  are shown disposed along the second fluid line  426 , and more specifically between line portion  466  and the pumps  432 ,  432 a. The pump damper chamber  436  and the orifice  454  can reduce the amplitude of pressure pulsations passing through the system  410 . Pressurized brake fluid is delivered to the line portion  466  by the pumps  432  and  432   a  through the damper chamber  436  and orifice  454 . Fluid is pressurized by the pumps  432 ,  432   a  and is therefore at a higher pressure in line portion  66  than in line portions  462 ,  464  during operation of the pumps  432 ,  432   a.    
      Each of the plurality of pumps  432 ,  432   a  defines a repeating operating cycle in which fluid is drawn into the pumps  432 ,  432   a  at a first pressure and is urged out of the pumps  432 ,  432   a  at a second, higher pressure. The operation of each pump  432 ,  432   a  is controlled such that the operating cycles of the pump are offset with respect to one another. For example, the fluid pump  432  can be urging pressurized fluid to the line portion  466  while the fluid pump  432   a  is drawing fluid from the line portion  464 .  
      The pumps  432 ,  432   a  can be sized similar to a single pump used in prior art systems and be modified to deliver an equivalent flow rate. For example, the pumps  432 ,  432   a  can be piston pumps and the stroke of the piston in each of the pumps  432 ,  432   a  can be approximately one-half the stroke of a piston of single pump. The single pump would generate greater displacements of fluid for each stroke as compared to each of the individual pumps  432 ,  432   a , resulting in relatively greater fluid pressures during each stroke. In other words, the single pump of the prior art system would generally generate half the pressure pulsations of the pair of pumps  432 ,  432   a , however, the amplitude of each pulsation would be greater than the amplitude of individual pulsations generated by each of the pumps  432 ,  432   a.    
      In operation, offsetting the operating cycles of the pumps  432 ,  432   a  substantially reduces the amplitude of fluid pressure pulsations passing through the system  410 , especially at a brake pedal  452  of the system  410 .  FIG. 12  is a graph schematically showing a first line or truncated wave  434  generally representing fluid pressure in the line portion  466  during operation of the system  410 . The x-axis demarcates time. The line  434  defines a plurality of cycles, each cycle starting when the line  434  is at a minimum pressure value and ending after the line  434  has reached a maximum pressure value and returned to the minimum pressure value. Every other cycle corresponds to the pressure increase in the fluid line  466  associated with one of the pumps  432 ,  432   a  discharging pressurized fluid to the line  466 . Adjacent cycles correspond to a first of the pumps  432 ,  232   a  discharging fluid and a second of the pumps  432 ,  432   a  discharging fluid.  
      In the prior art methods using a single pump, a graphical line representing pressure at the single pump outlet defines gaps between adjacent cycles since pressurized fluid is not delivered to the fluid line portion downstream of the single pump when the single pump is drawing fluid to be pressurized. In addition, the amplitude of a cycle in the prior art pressure graph is greater than the amplitude of the cycles defined by line  434  since the flow rate demanded of the prior art system must be satisfied by fewer pump discharges. In other words, the amplitude of the line  434  is reduced by the arrangement of a plurality of pumps  432 ,  432   a  arranged in parallel to one another. For example, the amplitude of a cycle of the line  434  is approximately one half of the amplitude of a cycle of a graphical line representing pressure in a prior art, single pump system.  
      A second line  434   a  represents the fluid pressure in the line portion  464  during operation and corresponds to vacuum created when the pumps  432 ,  432   a  draw fluid. Another benefit of the present invention is that vacuum at the inlet of the pumps  432 ,  432   a  is more consistent. The line  434   a  defines a plurality of cycles, each cycle starting when the line  434   a  is at a maximum pressure value and ending after the line  434   a  has reached a minimum pressure value and returned to the maximum pressure value. Every other cycle corresponds to the pressure decrease in the fluid line  464  associated with one of the pumps  432 ,  432   a  drawing fluid from the line  464 . Adjacent cycles correspond to a first of the pumps  432 ,  432   a  drawing fluid and a second of the pumps  432 ,  432   a  drawing fluid. At least one of the pumps  432 ,  432   a  is likely drawing fluid at all times. The wave  434   a  is closer to the x-axis since the negative pressure or vacuum in the line portion  464  is not as great as the pressure of fluid in the line portion  466 .  
      In the prior art methods using a single pump, a graphical line representing pressure at the single pump inlet defines gaps between adjacent cycles since fluid is not drawn from the fluid line portion upstream of the single pump when the single pump is discharging pressurized fluid. In addition, the amplitude of a cycle in the prior art pressure graph is greater than the amplitude of the cycles defined by line  434   a  since the flow rate demanded of the prior art system must be satisfied by fewer pump discharges. In other words, the amplitude of the line  434   a  is reduced by the arrangement of a plurality of pumps  432 ,  432   a  arranged in parallel to one another. For example, the amplitude of a cycle of the line  434   a  is approximately one half of the amplitude of a cycle of a graphical line representing pressure in a prior art, single pump system. Maintaining a more steady vacuum at the inlet of the pumps  432 ,  432   a , as provided by the present invention, substantially reduces energy losses associated with starting and stopping a fluid stream moving through the various fluid paths extending between the master cylinder  412  or reservoir  414  and the pumps  432 ,  432   a  which results in improved pump flows and operating efficiencies.  
      In one embodiment of the invention, the operating cycles are offset 180 degrees from one another. In other words, one of the pumps  432 ,  432   a  is drawing fluid while the other pump  432 ,  432   a  is urging fluid to the brakes  418 ,  420 . However the invention can be practiced wherein the operating cycles  434 ,  434   a  are offset less than 180 degrees from one another. The operating cycles of the plurality of pumps  432 ,  432   a  are controlled to minimize pressure pulsations.  
       FIG. 13  shows a second exemplary embodiment of the invention including three pumps  432   b ,  432   c ,  432   d . The brake system  410   a  includes a master cylinder  412   a  communicating with a reservoir  414   a . A first fluid line  416   a  extends between the reservoir  414   a  and one or more brakes  418   a ,  420   a  disposed at wheels  422   a ,  424   a . A second fluid line  426   a  extends between a first position  428   a  along the first fluid line  416   a  and a second position  430   a . The plurality of fluid pumps  432   b ,  432   c ,  432   d  are disposed in parallel with respect to one another along the second fluid line  426   a.    
      Each of the pumps  432   b ,  432   c ,  432   d  defines a repeating operating cycle in which fluid is drawn into the pump  432   b ,  432   c ,  432   d  at a first pressure and is urged out of the pump  432   b ,  432   c ,  432   d  at a second, higher pressure. The operating cycles of the pumps  432   b ,  432   c ,  432   d  can be offset 120° from one another. For example, two of the pumps  432   b ,  432   c ,  432   d  can be drawing fluid while the third of the pumps  432   b ,  432   c ,  432   d  can be urging fluid to the brakes  418   a ,  420   a . The operation of the pumps  432   b ,  432   c ,  432   d  can be controlled so that the fluid pressure in line portions  466   a ,  464   a  varies over time as shown by lines  434 ,  434   a , respectively, in  FIG. 12 .  
      The pumps  432   b ,  432   c ,  432   d  can be sized similar to a single pump used in prior art systems and be modified to deliver an equivalent flow rate. For example, the pumps  432   b ,  432   c ,  432   d  can be piston pumps and the stroke of the piston in each of the pumps  432   b ,  432   c ,  432   d  can be approximately one-third the stroke of a piston of single pump. The single pump would generate greater displacements of fluid for each stroke as compared to each of the individual pumps  432   b ,  432   c ,  432   d , resulting in relatively greater fluid pressures during each stroke. In other words, the single pump of the prior art system would generally generate one third of the pressure pulsations of the three pumps  432   b ,  432   c ,  432   d , however, the amplitude of each pulsation would be greater than the amplitude of the individual pressure pulsations generated by each of the pumps  432   b ,  432   c ,  432   d.    
      Referring again to  FIG. 11 , the invention also provides a third fluid line  438  to define a separate feed circuit to the pumps  432 ,  432   a  to enhance the operation of the system  410 . The fluid line  438  cap improve the cold temperature response of the system especially during braking operations in which the driver of the vehicle is not engaging the brake pedal and the pumps  432 ,  432   a  act as suction pump. The fluid line  38  can communicate fluid from the reservoir  414  to the line portion  464  at the first position  440  along -the second fluid line  426 . The fluid line  438  can be larger than the other fluid lines  416 ,  426  of the system to reduce the restriction acting against fluid movement between the reservoir  414  and the pumps  432 ,  432   a . By way of example and not limitation, the fluid line  438  can be a 10 millimeter hose and the other fluid line portions  460 ,  462 ,  464 ,  466  can be 6 millimeter brake lines.  
      A first prime valve  442  is disposed along the third fluid line  438  between the reservoir  414  and the first position  440 . In the exemplary embodiment of the invention, the valve  442  is a solenoid check valve set in a first position when de-energized to prevent fluid from moving to the reservoir  414 . The valve  442  can be selectively moved to a second position when energized to reduce the restriction acting against fluid movement from the reservoir  414  to the pumps  432 ,  432 a. A second prime valve  444  is disposed along the second fluid line  426  between the line portions  460 ,  462 . In the exemplary embodiment of the invention, the prime valve  444  is a solenoid check valve set in a first position when de-energized to prevent the high pressure of the master cylinder primary circuit from entering inlets to pumps  432 ,  432   a  in base brake operation. The valve  444  moves to the open position when energized to reduce the restriction acting against fluid movement from the line  416  to the pumps  432 ,  432   a . The first and second prime valves  442 ,  444  are energized during a controlled braking event to provide parallel flow paths to the inlets of pumps  432 ,  432   a . The first prime valve  442  can be larger than the second prime valve  444  for the same electrical energy consumption since it is only exposed to reservoir inlet pressures.  
      A controller  456  can control the motor  433  to control the operation of the pumps  432 ,  432 a. The controller  456  can also control the movement of the valves  442 ,  444 . The controller  456  can control the motor  433  and valves  442 ,  444 , in accordance with a program stored in memory to enhance the deceleration of the vehicle.  
      The present invention can also be used in a braking system having a Front/Front/Rear/Rear configuration. An embodiment of the invention used in combination with a Front/Front/Rear/Rear system would include a plurality of pumps disposed in each of the separate hydraulic circuits. The present invention can be used with any braking system having a pre-charge.  
      Having described the invention in detail and by reference to the preferred embodiments, it will be apparent that modifications and variations thereof are possible without departing from the scope of the invention.