Abstract:
A vehicular brake system having vehicle stability management includes a hydraulic master cylinder connected to wheel brakes via brake conduits. A pump generates fluid pressures and pressure control valves located between the master cylinder and the wheel brakes regulate the fluid pressures at the wheel brakes to achieve ABS and traction control. A medium pressure accumulator stores fluid pressurized by the pump which is supplied to the wheel brakes via associated control valves to achieve VSM braking control. The brake system has low power requirements because the medium pressure accumulator does not have to be filled quickly, yet the stored pressurized fluid can be released to the wheel brakes to quickly produce the braking pressures necessary for initiating most VSM applications. The pump is used to supplement the accumulator pressures to achieve full VSM control.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This claims the benefit of U.S. provisional patent application identified as application No. 60/032,872, filed Dec. 13, 1996, and a CIP of PCT/US97/23022, filed Dec. 12, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates in general to a vehicular brake system. In particular, this invention relates to a vehicle stability management (VSM) system for use in an anti-lock brake (ABS) and traction control (TC) brake system. 
     Vehicles are commonly slowed and stopped with hydraulic brake systems. While these systems vary in complexity, a typical base brake system includes a tandem master cylinder, fluid conduit arranged in two similar but separate brake circuits, and wheel brakes in each circuit. The master cylinder generates hydraulic forces in both brake circuits by pressurizing brake fluid when the driver steps on the brake pedal. The pressurized fluid travels through the fluid conduit in both circuits to actuate brake cylinders at the wheels and slow the vehicle. 
     Braking a vehicle in a controlled manner under adverse conditions requires precise application of the brakes by the driver. Under these conditions, a driver can easily apply excessive brake pressure thus causing one or more wheels to lock, resulting in excessive slippage between the wheel and road surface. Such wheel lock-up conditions can lead to greater stopping distances and possible loss of directional control. 
     Advances in braking technology have led to the introduction of ABS systems. An ABS system monitors wheel rotational behavior and selectively applies and relieves brake pressure in the corresponding wheel brakes in order to maintain the wheel speed within a selected slip range while achieving maximum braking forces. While such systems are typically adapted to control the braking of each braked wheel of the vehicle, some systems have been developed for controlling the braking of only a portion of the braked wheels. 
     Electronically controlled ABS valves, comprising apply (isolation) valves and dump valves, are located between the master cylinder and the wheel brakes and perform the pressure regulation. Typically, when activated, these ABS valves operate in three pressure control modes: pressure apply, pressure dump and pressure hold. The apply valves allow brake pressure into the wheel brakes to increase pressure during the apply mode, and the dump valves release pressure from the wheel cylinders during the dump mode. Wheel cylinder pressure is held constant during the hold mode. 
     A further development in braking technology has led to the introduction of traction control (TC) systems. Additional valves have been added to existing ABS systems to provide a brake system which controls wheel speed during acceleration. Excessive wheel speed during vehicle acceleration leads to wheel slippage and a loss of traction. An electronic control system senses this condition and automatically applies braking pressure to the wheel cylinders of the slipping wheel to reduce the slippage and increase the traction available. In order to achieve optimal vehicle acceleration, braking pressures greater than the master cylinder pressure must quickly be available when the vehicle is accelerating. 
     During vehicle motion such as cornering, dynamic forces are generated which can reduce vehicle stability. A VSM brake system improves the stability of the vehicle by counteracting these forces through selective brake actuation. These forces and other vehicle parameters are detected by sensors which signal an electronic control unit. The electronic control unit automatically operates pressure control devices to regulate the amount of hydraulic pressure applied to specific individual wheel brakes. In order to achieve optimum vehicle stability, brake pressures greater than the master cylinder pressure may be required in a very short time. However, a brake system that generates high pressures very quickly typically has high power requirements or uses a large high pressure accumulator. 
     It would be desirable to provide an ABS/TC/VSM brake system which would provide fluid pressures in excess of master cylinder pressure quickly using a low amount of power and a low amount of stored energy. 
     SUMMARY OF THE INVENTION 
     This invention relates to an improved ABS/TC/VSM vehicle brake system. The vehicle brake system includes a hydraulic master cylinder connected to wheel brakes via brake conduits. A pump generates fluid pressures and pressure control valves located between the master cylinder and the wheel brakes regulate the fluid pressures at the wheel brakes to achieve ABS and traction control. A medium pressure accumulator stores fluid pressurized by the pump which is supplied to the wheel brakes via associated control valves to achieve VSM braking control. The brake system has low power requirements because the medium pressure accumulator does not have to be filled quickly, yet the stored pressurized fluid can be released to the wheel brakes to quickly produce the braking pressures necessary for initiating most VSM applications. The pump is used to supplement the accumulator pressures to achieve full VSM control. 
     Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a hydraulic circuit schematic of an ABS/TC/VSM brake system with a medium pressure accumulator having two channel VSM control in accordance with this invention. 
     FIG. 2 is a hydraulic circuit schematic of an ABS/TC/VSM brake system with medium pressure accumulators having four channel VSM control in accordance with this invention. 
     FIG. 3 is sectional view of a medium pressure accumulator illustrated schematically in the circuit FIG.  1 . 
     FIG. 4 is sectional view of a bypass valve illustrated schematically in the circuit FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an ABS/TC/VSM brake system  10  according to this invention. The brake system  10  includes a tandem master cylinder  12  for pressurizing brake fluid when the driver steps on the brake pedal  14 . A brake switch  16  is connected to the Electronic Control Unit (ECU)  18  to indicate that the driver is stepping on the brake pedal  14 . A reservoir  20  is connected to the master cylinder  12  and holds a supply of brake fluid at atmospheric pressure. Two separate brake circuits  22   a,    22   b  are connected to the master cylinder  12  via main fluid conduits  24  and  26  respectively. The brake system  10  is preferably configured as a vertical split system with brake circuit  22   a  having first and second wheel brakes  28  and  29  connected to the master cylinder  12  via the main conduit  24  and brake circuit  22   b  having first and second wheels brakes  30  and  31  connected to the master cylinder  12  via main conduit  26 . The brake system  10  provides ABS control to all four wheel brakes  28 - 31  and brake circuit  22   b  provides VSM and traction control to the wheel brakes  30  and  31 . 
     In brake circuit  22   a,  the main conduit  24  splits into two conduits  32  and  33 . A normally open solenoid actuated 2-position, 2-way ABS isolation valve  34  is located in conduit  32  between the master cylinder  12  and the wheel brakes  28  and  29 . The solenoid actuated isolation valve  34  has a first, open position  34   a  and a second position  34   b  having a one-way valve which allows fluid to flow from the wheel brakes  28  and  29  towards the master cylinder  12  but prevents flow in the opposite direction. A pump  36  having an inlet  36   a  and an outlet  36   b  is located in conduit  33 . A 2-position, 2-way solenoid actuated dump valve  38  is located in conduit  33  between the wheel brakes  28  and  29  and the pump inlet  36   a.  A damping chamber  37  and restricting orifice  39  are located at the pump outlet  36   b  to reduce the pressure pulsations from the pump. A low pressure accumulator (LPA)  40  is located in conduit  33  between the pump  36  and the dump valve  38 . The dump valve  38  has a first, one-way position  38   a  which prevents fluid from flowing from the wheel brakes  28  and  29  to the LPA  40  but allows fluid to flow in the opposite direction, and a second, open position  38   b  allowing flow in both directions. 
     In circuit  22   b,  a master cylinder pressure transducer  41  is located in conduit  26  and is connected to the ECU  18  to indicate the master cylinder pressure. The main brake conduit  26  splits into two conduits  42  and  43 . Conduit  42  is connected to the first wheel brake  30  and conduit  43  is connected to the second wheel brake  31 . A first normally open solenoid actuated 2-position, 2-way ABS isolation valve  44  is located in conduit  42  between the first wheel brake  30  and the master cylinder  12 . A second normally open solenoid actuated 2-position, 2-way ABS isolation valve  46  is located in conduit  43  between the second wheel brake  31  and the master cylinder  12 . The ABS isolation valves  44 ,  46  have a first open position  44   a,    46   a  and a second position  44   b,    46   b  having a one-way valve which allows fluid to flow from the wheel brakes  30  and  31  towards the master cylinder  12  but prevents flow in the opposite direction. A normally open solenoid actuated 2-position, 2-way traction control isolation valve  48  is located in conduit  26  between the master cylinder  12  and the ABS isolation valves  44  and  46 . The traction control isolation valve  48  has a first open position  48   a,  and a second position  48   b  having a one-way valve which allows fluid to flow from the master cylinder  12  towards the wheel brakes  30  and  31  but prevents flow in the opposite direction. 
     Conduits  50  and  51  connect the first and second wheel brakes  30  and  31  respectively to a conduit  52  which is connected to conduit  43 . A pump  54  having an inlet  54   a  and an outlet  54   b  is located in conduit  52 . A damping chamber  55  and restricting orifice  57  are located at the pump outlet  54   b  to reduce the pressure pulsations from the pump  54 . A first 2-position, 2-way solenoid actuated dump valve  56  is located in conduit  50  between the wheel brake  30  and the connection with conduit  52 . A second 2-position, 2-way solenoid actuated dump valve  58  is located in conduit  51  between the wheel brake  31  and the connection with conduit  52 . A low pressure accumulator (LPA)  60  is located in conduit  52  between the pump  54  and the dump valves  56  and  58 . The dump valves  56 ,  58  have a first, one-way position  56   a,    58   a  which prevents fluid from flowing from the wheel brakes  30  and  31  to the LPA  60  but allows fluid to flow in the opposite direction, and a second, open position  56   b,    58   b  allowing flow in both directions. 
     A supply conduit  62  is connected to the main brake conduit  26  between the traction control isolation valve  48  and the master cylinder  12 . Fluid can flow from the master cylinder  12  through the main brake conduit  26  to reach the supply conduit  62  without traveling through a valve element. The supply conduit  62  is also connected to the pump inlet  54   a  for supplying the pump  54  with fluid. A 2-position, 2-way solenoid actuated supply valve  64  is located in the supply conduit  62  between the master cylinder  12  and the pump inlet  54   a.  The supply valve  64  has a first, one-way position  64   a,  in which a spring-loaded check valve  65  prevents fluid from flowing from the master cylinder  12  to the pump inlet  54   a  but allows fluid to flow in the opposite direction when the fluid reaches pressures of approximately 800 p.s.i. greater than the master cylinder pressure. The 800 p.s.i. pressure requirement may be different depending on system parameters. The supply valve  64  also has a second, open position  64   b  allowing flow in both directions. A one-way check valve  63  is located between the connection of the supply conduit  62  to conduit  52  and the LPA  60 . The check valve  63  prevents fluid in the supply conduit  62  from flowing into the LPA  60 , but allows fluid in the LPA  60  to flow towards the pump inlet  54   a.    
     A medium pressure accumulator (MPA)  66  is located in conduit  68  which connects conduit  62  to conduit  43 . The MPA  66  stores fluid at pressures which are higher than a typical low pressure accumulator but which are lower than a typical high pressure accumulator. The MPA  66  preferably stores fluid between 40 p.s.i. and 400 p.s.i., however fluid may be stored at other suitable pressures. A switch  69  on the MPA  66  is connected to the ECU  18  to indicate whether or not the MPA is full of pressurized fluid. 
     A first control valve in the form of a 2-position, 2-way solenoid actuated priming valve  70  is located in circuit  68  between its connection to the supply conduit  62  and the MPA  66 . The priming valve  70  has a first, one-way position  70   a,  in which a spring-loaded check valve  71  prevents fluid from flowing from the master cylinder  12  to the MPA  66  but allows fluid to flow in the opposite direction when the fluid reaches a pressure differential of approximately 1600 p.s.i. across the valve  71 . The priming valve  70  also has a second, open position  70   b  allowing flow in both directions. 
     A second control valve in the form of a 2-position, 2-way solenoid actuated charging valve  72  is located in circuit  68  between the connection with conduit  43  and the MPA  66 . The charging valve  72  has a first, one-way position  72   a,  in which a spring-loaded check valve  73  prevents fluid from flowing from the MPA  66  towards the wheel brakes  30  and  31  but allows fluid to flow in the opposite direction when the fluid reaches a pressure differential of approximately 1600 p.s.i. across the valve. The 1600 p.s.i. pressure requirements needed to open the spring loaded check valves  71  and  73  may be different values depending on system parameters. The charging valve  72  also has a second, open position  72   b  allowing flow in both directions. A switchable solenoid valve is used rather than a check valve because by opening the charging valve  72  the MPA  66  can be charged by the pump  54  without creating a large load on the pump  54 . Also, a solenoid valve is contamination resistant in the fully open position than a spring loaded check valve used as a relief valve. 
     A bypass valve  74  is connected to conduits  43  and  62  and is connected in parallel to the traction control isolation valve  48 . The bypass valve  74  prevents excessive pressure buildup by opening at approximately 2500 p.s.i. to allow pressurized fluid to flow back to the master cylinder  12  when the traction control isolation valve  48  is in the second position  48   b.  The opening pressure of the bypass valve  74  should be higher than the sum of the opening pressure of the spring loaded check valve  73  in the charging valve  72  plus the MPA pressure to keep fluid taken from the MPA  66  during VSM mode in the braking system (where it will be returned to the MPA) rather than being returned to the master cylinder  12 . 
     During normal braking the driver actuates the base braking system by pushing on the brake pedal  14  which causes the master cylinder  12  to pressurize brake fluid. In circuit  22   a,  the pressurized brake fluid travels through conduits  24  and  32 , through the open ABS isolation valve  34  and into the wheel brakes  28  and  29  to brake the vehicle. In circuit  22   b,  the pressurized brake fluid travels through conduits  26 ,  42  and  43 , through the open ABS isolation valves  44  and  46  and into the wheel brakes  30  and  31  to brake the vehicle. When the driver releases the brake pedal, the master cylinder  12  no longer pressurizes the brake fluid and the brake fluid returns to the master cylinder  12  via the same route. 
     During ABS modes, the driver applies the brakes in a similar manner as during normal braking. When a wheel begins to slip, the pumps  36  and  54  run and pressurize fluid in circuits  22   a  and  22   b.  The ABS isolation valves  34 ,  44  and  46  and the ABS dump valves  38 ,  56  and  58  are pulsed to control the pressures at the wheel brakes  28 ,  29 ,  30 , and  31 . 
     The MPA  66  is filled, or charged, with pressurized fluid during a charging mode. The charging mode is initiated when the MPA switch  69  indicates that the MPA  66  is not full and the brake switch  16  and master cylinder pressure transducer  41  indicate that the driver is not requesting base braking by pushing on the brake pedal  14 . The traction control isolation valve  48 , and the first and second ABS isolation valves  44  and  46 , are shuttled to their second positions  48   b,    44   b,  and  46   b  to prevent pressurized fluid from reaching the master cylinder  12  and wheel brakes  30  and  31 . The charging valve  72  is shuttled to the second position  72   b  to open a path between the pump outlet  54   b  and the MPA  66 . The supply valve  64  is shuttled to the second position  64   b  to allow fluid from the master cylinder  12  to supply the pump inlet  54   a.  The pump  54  runs and pumps pressurized fluid into the MPA  66  until the MPA switch  69  indicates that the MPA  66  is full. When the MPA  66  is full, the pump  54  is turned off and the traction control isolation valve  48 , ABS isolation valves  44  and  46 , supply valve  64  and charging valve  72  are returned to the first position  48   a,    44   a,    46   a,    64   a  and  72   a.  The pressure of the fluid stored in the MPA  66  when it is full is approximately 400 p.s.i., although any suitable pressure can be used. 
     The spring loaded check valve  71  in the priming valve  70  provides a pressure relief function which prevents fluid expansion in a fully charged MPA from generating pressures capable of damaging components. For example, if the temperature of the fluid in the fully charged MPA  66  should increase, the pressure in the MPA  66  will increase. The increased pressure will open the check valve  71  and the excess fluid will flow back to the master cylinder  12  through the check valves (not shown) located in the pump  54 . 
     The brake system  10  provides VSM control to the wheel brakes  30  and  31  using circuit  22   b  to generate the necessary fluid pressures. VSM control may be needed when the driver is applying the brakes or when the driver is not applying the brakes. Pressurized fluid stored in the MPA  66  is used to provide fluid flow rates which are greater than those available from a standard ABS/TC pump  54  to begin to fill the wheel brakes  30 ,  31 . When VSM control is needed, the charging valve  72  is switched to the open position  72   b  and pressurized fluid flows from the MPA  66  towards the isolation valves  44  and  46  which are selectively pulsed open to allow fluid into the affected wheel  30 ,  31 . Alternatively, the priming valve  70  could be switched to the open position  70   b  to allow pressurized fluid to flow from the MPA  66  through the pump  54  to the wheel brakes  30 ,  31  but this path includes restrictions which would limit the flow. When the MPA  66  has discharged to a pressure below a predetermined pressure, the charging valve  72  is switched back to the one-way position  72   a.  The priming valve  70  is switched to the open position  70   b  and the pressurized fluid still in the MPA  66  is supplied to the pump inlet  54   a  which improves the pump efficiency. The pump  54  pumps more pressurized fluid towards the wheel brakes  30 ,  31 , and VSM braking pressures are achieved by pulsing the isolation valves  44 ,  46  and dump valves  56 ,  58  to regulate the pressures at the wheel brakes  30 ,  31 . 
     The valves and pumps are preferably mounted together in a hydraulic control unit (not shown). The hydraulic control unit may be mounted in a remote location using longer conduits to connect it with the master cylinder  12 . The longer conduits typically impart flow restrictions which lengthen the time required to charge the MPA  66 , however, the time required to charge the MPA  66  is not critical. 
     During traction control or when VSM control is needed while the driver is not pushing the brake pedal the traction control isolation valve  48  is shuttled to the second position  48   b  to prevent the pressurized fluid from reaching the master cylinder  12 . The first and second ABS isolation valves  44  and  46  are also shuttled to the second positions  44   b  and  46   b  to prevent pressurized fluid from reaching the wheel brakes  30  and  31 . The pump  54  runs and pressurizes fluid. The ECU  18  selects the wheel to be braked and pressurized fluid is supplied to it by shuttling the charging valve  72  to the second, open position  72   b  and pulsing the corresponding ABS isolation valve  44  or  46  to the second, open position  44   b  or  46   b.  The pressurized fluid in the MPA  66  flows into the selected wheel brake  30  or  31  providing a rapid pressure increase. The charging valve  72  is shuttled back to the first position  72   a  and further pressure is applied by pulsing the priming valve  70  to the second, open position  70   b  to feed the pump inlet  54   a  with pressurized fluid from the MPA  66 . The spring loaded check valve  65  in the supply valve  64  holds pressure on the pump inlet  54   a  side of the supply valve  64  so that the fluid released from the MPA  66  by the priming valve  70  will not flow back to the master cylinder  12 . 
     The pressure at the selected wheel brake  30  or  31  is increased in a controlled manner by pulsing the corresponding ABS isolation valve  44  or  46  open and closed. The pressure is decreased in a controlled manner by pulsing open the corresponding ABS dump valve  56  and  58 , allowing some of the pressurized fluid in the wheel brake  30  or  31  to flow into the LPA  60 . While the ABS isolation valve  44  or  46  is pulsed closed, the pressurized fluid in the LPA  60  is pumped through the spring loaded check valve  73  in the charging valve  72  to charge the MPA  66 . Therefore, the amount of fluid stored in the LPA  60  is minimized to provide adequate storage requirements in case of transition to ABS. In addition, the amount of fluid stored in the MPA  66  is maximized to reduce the need to enter the MPA charging mode. 
     If the driver should apply the brakes during the TC or VSM mode just described (VSM without brake apply), some pedal movement will be experienced as the master cylinder  12  pressurizes the brake fluid in circuit  22   a.  However, the drive is isolated from the front wheel brakes  30  and  31  and some action must be taken in circuit  22   b  or the driver will experience an unusually high, hard brake pedal  14 . When the pressure transducer  41  and the brake switch  16  indicate that the driver is applying the brakes during TC or VSM mode, the priming valve  70  remains in the first position  70   a  and the supply valve  64  is shuttled to the second position  64   b.  The pressurized fluid from the master cylinder  12  is supplied to the pump inlet  54   a  and the driver will experience brake pedal movement that is typical to normal base braking. When the MPA switch  69  indicates to the ECU  18  that the MPU  66  is full, the supply valve  64  is returned to the first position  64   a.    
     When VSM mode is entered while the driver is already applying the brakes, the valve control is the same as in VSM without brake pedal apply except that the supply valve  64  is pulsed to the second, open position  64   b  instead of the priming valve  70 . The driver will experience brake pedal movement typical of normal base braking and the pump inlet  54   a  is supplied with fluid. Further VSM control is similar to the VSM control without brake pedal apply described above. When the driver releases the brake pedal  14 , the excess fluid in circuit  22   b  which was supplied by the master cylinder  12  is pumped back to the master cylinder  12  through the bypass valve  74 . Since the master cylinder pressure may be at a relatively high pressure, the bypass valve  74  references atmospheric pressure and opens when the pressure at the pump outlet  54   b  reaches approximately 2500 p.s.i. above atmospheric pressure. 
     During a transition from ABS control to VSM control the traction control isolation valve  48  is shuttled to the second position  48   b  to allow pressures greater than master cylinder pressure to be achieved at the wheel brakes  30  and  31 . Fluid may still be stored in the LPA  60  from the previous ABS mode, and this fluid is pumped through the bypass valves  74  and back to the master cylinder  12 . Through proper control of the valves and utilizing information from the MPA switch  69 , a consistent relationship of pedal travel to brake pressure can be maintained in all modes of operation. 
     A second embodiment of a brake system according to this invention is indicated generally at  110  in FIG.  2 . The brake system  110  includes two similar but separate brake circuits  122   a  and  122   b  connected to the master cylinder  112  via respective main conduits  124  and  126 . The brake system  110  man be configured as a diagonally split system (not illustrated) with brake circuit  122   a  including a first driven wheel brake  128  and a first non-driven wheel brake  129 , and brake circuit  122   b  including a second driven wheel brake  130  and a second non-driven wheel brake  131 . Alternatively, the brake system  110  may be configured as a vertically split system with brake circuit  122   a  including first and second non-driven wheel brakes  128  and  129 , and brake circuit  122   b  including first and second driven wheels brakes  130  and  131  as illustrated in FIG.  2 . 
     Both brake circuits  122   a  and  122   b  include the same components as circuit  22   b  of FIG.  1 . The brake circuits  122   a  and  122   b  also operate in an identical manner as circuit  22   b  of FIG. 1 to provide selective ABS, traction control and VSM control to all four wheel brakes  128 - 131  individually. 
     Referring now to FIG. 3, there is illustrated a medium pressure accumulator indicated generally at  66  according to this invention. The MPA  66  includes a housing  268  having a bore  270 . A port  271  intersects the bore  270  and connects with conduit  68  shown in FIG. 1. A cup-shaped end cap  272  is disposed within the bore  270  and secured by a snap ring  285 . The cup-shaped end cap  272  includes an annular rim surface  273  which extends into the bore  270 . The end cap  272  includes a seal  274  to sealingly enclose the bore  270  to keep out contaminants. A cup-shaped piston  276  is slidably disposed within the bore  270  and includes an annular rim  277  which extends into the bore  270 . A seal  278  is disposed within a groove  279  in the outer surface of the piston  276 . A pressure chamber  280  is defined between the sealed piston  276  and port  271 . A cylinder piston extension  281  having a shoulder  282  is disposed within the cup-shaped piston. A spring  284  is disposed between the piston  276  and the end cap  271 . The spring  284  abuts the shoulder  282  of the extension  281  and biases the extension  281  against the piston  276  and also biases the piston  276  towards the port  271 . A switch  269  is mounted to the end cap  272  and includes an extension  286  which extends into the bore  270  and past the end cap. 
     The MPA  66  stores pressurized fluid in the pressure chamber  280 . Fluid entering the pressure chamber  280  from port  271  pushes the piston  276  upwards towards the end cap  272  and expands the pressure chamber  280 . The spring  284  exerts a force against the piston  276  which pressurizes the fluid in the pressure chamber  280 . When the MPA  66  begins to fill, the fluid pressure in the pressure chamber  280  is approximately 40 p.s.i. When the MPA  66  is full, the piston  276  contacts the end cap  272  and the annular rim  277  of the piston  276  abuts the annular rim  273  of the end cap  272 . Also, the extension  281  abuts the switch extension  286  which trips the switch  269  indicating that the MPA  66  is full. When the MPA  66  is full, the fluid pressure in the pressure chamber  280  is approximately 400 p.s.i. When fluid exists the pressure chamber  280 , the piston  276  moves downwardly and the piston extension  281  no longer contacts the switch extension  286  and the switch  269  indicates that the MPA  66  is no longer full. 
     Referring now to FIG. 4, there is illustrated the bypass valve indicated generally at  74  according to the invention. The bypass valve  74  includes a housing  302  having a bore  304 . A first port  306  connected with conduit  43  intersects the bore  304 , and a second port  308  connected with conduit  62  intersects the bore  304 . A filter, preferably a cigar band-type filter  307 , is disposed at the first port  306 . A sleeve  310  is disposed within the bore  304  and secured therein by a snap ring  312 . A first sleeve seal  314  is disposed between the outer surface of the sleeve  310  and the bore  304  to prevent fluid flow between the first and second ports  306  and  308 . The first sleeve seal  314  is preferably a lip seal which may allow some fluid flow from the second port  308  to the first port  306  but not in the opposite direction; however, other known seals may be used. A second sleeve seal  316  is disposed between the outer surface of the sleeve  310  and the bore  304  to prevent fluid flow between the first port  306  and the atmosphere. The sleeve  310  includes a coaxial bore  318  having a first sleeve shoulder  320  and a second sleeve shoulder  322 . A radial bore  324  intersects the sleeve coaxial bore  318  providing fluid communication between the first port  306  and the coaxial bore  318 . An end piece  326  is disposed in the bore  304  and retained therein by a swage  328  formed on the sleeve  310 . A seal  330  is disposed in a groove  332  formed on the outer surface of the end piece  326 . The end piece  326  includes a coaxial bore  334  having a valve seat  336 . An optional orifice  337  is disposed beneath the valve seat  336  which improves the contamination resistance of the valve by creating greater valve lift. An optional filter  338  is disposed in the end piece coaxial bore  334 . 
     A poppet  340  is slidably disposed within the sleeve coaxial bore  318  coaxial to the end piece  326 . The poppet  340  includes a first end  342  having a shoulder  344  and a coaxial bore  346 . A check element, such as a ball  347 , is disposed in the poppet bore  346  for seating against the valve seat  336 . The poppet  340  further includes a second end  348  which is sealed by seal  350  abutting the second sleeve shoulder  322  to prevent fluid flowing from the sleeve coaxial bore  318  to the atmosphere. An annular washer  352  is disposed against the first sleeve shoulder  320  and a spring  354  is disposed between the washer  352  and the poppet shoulder  344 . The spring  354  biases the poppet  340  towards the end piece  326  so that the ball  347  seats against the valve seat  336  and closes fluid communication between the first and second ports  306  and  308 . 
     When the fluid pressure at port  306  and inside the sleeve bore  318  reaches a predetermined pressure, the poppet  340  is pushed upward and the ball  347  moves off the valve seat to allow fluid to flow through the bypass valve  74  from port  306  to port  308 . The fluid pressure require to lift the poppet  340  and open the bypass valve  74  is preferably approximately 2500 p.s.i., but may be any suitable pressure. The poppet seal  350  allows the poppet  340  to be referenced to atmosphere so that the fluid pressure required lift the poppet  340  is measured relative to atmospheric pressure. 
     In accordance with the provisions of the patent statutes, the principal and mode of operation of this invention have been described and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.