Patent Publication Number: US-11643062-B2

Title: Vehicle brake system and diagnostic method for determining a leak in one or more three-way valves

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to a vehicle brake system and a diagnostic system for determining the existence of a leak in one or more three-way valves. 
     BACKGROUND 
     Vehicles are commonly slowed and stopped with hydraulic brake systems. These systems vary in complexity but a base brake system typically includes a brake pedal, a tandem master cylinder, fluid conduits arranged in two similar but separate brake circuits, and wheel brakes in each circuit. The driver of the vehicle operates a brake pedal which is connected to the master cylinder. When the brake pedal is depressed, the master cylinder generates hydraulic forces in both brake circuits by pressurizing brake fluid. The pressurized fluid travels through the fluid conduit in both circuits to actuate brake cylinders at the wheels to slow the vehicle. 
     Base brake systems typically use a brake booster which provides a force to the master cylinder which assists the pedal force created by the driver. The booster can be vacuum or hydraulically operated. A typical hydraulic booster senses the movement of the brake pedal and generates pressurized fluid which is introduced into the master cylinder. The fluid from the booster assists the pedal force acting on the pistons of the master cylinder which generate pressurized fluid in the conduit in fluid communication with the wheel brakes. Thus, the pressures generated by the master cylinder are increased. Hydraulic boosters are commonly located adjacent the master cylinder piston and use a boost valve to control the pressurized fluid applied to the booster. 
     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 braking 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 Anti-lock Braking Systems (ABS). 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 to achieve maximum braking force. 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 plurality of braked wheels. 
     Electronically controlled ABS valves, comprising apply valves and dump valves, are located between the master cylinder and the wheel brakes. The ABS valves regulate the pressure between the master cylinder and the wheel brakes. Typically, when activated, these ABS valves operate in three pressure control modes: pressure apply, pressure dump and pressure hold. The apply valves allow pressurized brake fluid into respective ones of the wheel brakes to increase pressure during the apply mode, and the dump valves relieve brake fluid from their associated wheel brakes during the dump mode. Wheel brake pressure is held constant during the hold mode by closing both the apply valves and the dump valves. 
     To achieve maximum braking forces while maintaining vehicle stability, it is desirable to achieve optimum slip levels at the wheels of both the front and rear axles. During vehicle deceleration different braking forces are required at the front and rear axles to reach the desired slip levels. Therefore, the brake pressures should be proportioned between the front and rear brakes to achieve the highest braking forces at each axle. ABS systems with such ability, known as Dynamic Rear Proportioning (DRP) systems, use the ABS valves to separately control the braking pressures on the front and rear wheels to dynamically achieve optimum braking performance at the front and rear axles under the then current conditions. 
     A further development in braking technology has led to the introduction of Traction Control (TC) systems. Typically, 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, pressurized brake fluid is made available to the wheel cylinders even if the master cylinder is not actuated by the driver. 
     During vehicle motion such as cornering, dynamic forces are generated which can reduce vehicle stability. A Vehicle Stability Control (VSC) 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 optimal vehicle stability, braking pressures greater than the master cylinder pressure must quickly be available at all times. 
     Brake systems may also be used for regenerative braking to recapture energy. An electromagnetic force of an electric motor/generator is used in regenerative braking for providing a portion of the braking torque to the vehicle to meet the braking needs of the vehicle. A control module in the brake system communicates with a powertrain control module to provide coordinated braking during regenerative braking as well as braking for wheel lock and skid conditions. For example, as the operator of the vehicle begins to brake during regenerative braking, electromagnet energy of the motor/generator will be used to apply braking torque (i.e., electromagnetic resistance for providing torque to the powertrain) to the vehicle. If it is determined that there is no longer a sufficient amount of storage means to store energy recovered from the regenerative braking or if the regenerative braking cannot meet the demands of the operator, hydraulic braking will be activated to complete all or part of the braking action demanded by the operator. Preferably, the hydraulic braking operates in a regenerative brake blending manner so that the blending is effectively and unnoticeably picked up where the electromagnetic braking left off. It is desired that the vehicle movement should have a smooth transitional change to the hydraulic braking such that the changeover goes unnoticed by the driver of the vehicle. 
     Some braking systems are configured such that the pressures at each of the wheel brakes can be controlled independently (referred to as a multiplexing operation) from one another even though the brake system may include a single source of pressure. Thus, valves downstream of the pressure source are controlled between their open and closed positions to provide different braking pressures within the wheel brakes. Such multiplex systems, which are all incorporated by reference herein, are disclosed in U.S. Pat. No. 8,038,229, U.S. Patent Application Publication No. 2010/0016083, U.S. Patent Application Publication No. 2012/0013173, and U.S. Patent Application Publication No. 2012/0136261. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY 
     In a first embodiment of the present disclosure, a diagnostic method for a vehicle brake system is provided which identifies a leak in a three-way valve within the vehicle brake system. The diagnostic method includes the steps of: (1) providing a pedal simulator having a pressure medium within a simulation chamber of the pedal simulator; (2) de-energizing a secondary three-way valve; (2) retracting a plunger to a home position within a plunger assembly to reduce boost circuit pressure to zero while also monitoring the pressure at an output of a fluid separator via a secondary master cylinder pressure sensor; (3) determining a rate of pressure reduction at the output of a fluid separator via the secondary master cylinder pressure sensor; and (4) identifying a leak in at least one of a primary three-way valve and the secondary three-way valve if the rate of pressure reduction is equal to or greater than a pre-determined rate. The pre-determined rate may, but not necessarily, be defined to be about 7 bar/100 msec. 
     The vehicle brake system which implements the aforementioned diagnostic method may include a reservoir and a master cylinder which are disposed in a first module while the pedal simulator, a simulator test valve, and the plunger assembly of the vehicle brake system are disposed in a second module which is separate from the first module. As a result of the dual module design having a remote master cylinder in the first module, this vehicle brake system may be easier to package within a vehicle due to space limitations in a vehicle. Moreover, the vehicle brake system of the present disclosure includes a primary three-way valve and a secondary three-way valve. The primary three-way valve is in fluid communication with a second wheel brake and a third brake while the secondary three-way valve is in fluid communication with a first wheel brake and a fourth wheel brake. 
     With respect to the diagnostic method provided above, it is understood that the step of identifying a leak in at least one of the primary and secondary three-way valves may be performed via a signal transmitted from the ECU to a vehicle user interface. Moreover, the step of providing a pedal simulator having pressure medium disposed with a chamber of the pedal simulator, may further include the steps of: (1) energizing a pumping valve, the secondary three-way valve, and a plurality of apply valves disposed in the second module; (2) energizing a simulator valve disposed within the second module to enable bi-directional flow of pressure medium within the simulator valve; (3) applying and retracting the plunger in the plunger assembly while keeping the simulator valve energized, until pressure within a second output pressure chamber of the master cylinder reaches a pre-determined level; and (4) de-energizing the simulator valve while completing the stroke of the plunger in the plunger assembly once pressure in the second output pressure chamber of the master cylinder reaches the predetermined level. The pre-determined pressure level for the second output pressure chamber may, but not necessarily be about 1.5 bar. 
     The plunger assembly includes a motor which is actuated by an electronic control module and the motor causes the plunger in the plunger assembly to cycle within a first pressure chamber in the plunger assembly. Moreover, the second module of the aforementioned vehicle brake system houses every hydraulic valve of the vehicle brake system. Lastly, the pressure medium implemented in the aforementioned method and system may, but not necessarily, be brake fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present disclosure will be apparent from the following detailed description, best mode, claims, and accompanying drawings in which: 
         FIG.  1 A  is a first schematic diagram of a first braking system according to the present disclosure; 
         FIG.  1 B  is a second schematic diagram of the first braking system in  FIG.  1 A  wherein the simulator piston moves rear/right-ward as the plunger in the plunger assembly advances (moves forward/left) and applies pressure to the boost circuit; 
         FIG.  2 A  is a schematic diagram of a second braking system according to the present disclosure where the simulator is filled with pressure medium; and 
         FIG.  2 B  is a schematic diagram of a second braking system according to the present disclosure where the simulator emptied of pressure medium and the piston is advanced within the simulator. 
     
    
    
     Like reference numerals refer to like parts throughout the description of several views of the drawings. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present disclosure, which constitute the best modes of practicing the present disclosure presently known to the inventors. The figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the present disclosure that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the present disclosure and/or as a representative basis for teaching one skilled in the art to variously employ the present disclosure. 
     Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the present disclosure. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the present disclosure implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property. 
     It is also to be understood that this present disclosure is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present disclosure and is not intended to be limiting in any manner. 
     It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components. 
     The term “comprising” is synonymous with “including,” “having,” “containing,” or “characterized by.” These terms are inclusive and open-ended and do not exclude additional, unrecited elements or method steps. 
     The phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. When this phrase appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. 
     The phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps, plus those that do not materially affect the basic and novel characteristic(s) of the claimed subject matter. 
     The terms “comprising”, “consisting of”, and “consisting essentially of” can be alternatively used. Where one of these three terms is used, the presently disclosed and claimed subject matter can include the use of either of the other two terms. 
     Throughout this application, where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this present disclosure pertains. 
     The following detailed description is merely exemplary in nature and is not intended to limit the present disclosure or the application and uses of the present disclosure. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
     Referring now to the drawings, there is schematically illustrated in  FIGS.  1 A- 1 B  a first embodiment of a vehicle brake system, indicated generally at  10 . All valves of the brake system  10  are entirely disposed within the HCU which is configured to drive valves. The brake system  10  is a hydraulic boost braking system in which boosted fluid pressure is utilized to apply braking forces for the brake system  10 . The brake system  10  may suitably be used on a ground vehicle such as an automotive vehicle having four wheels with a wheel brake associated with each wheel. Furthermore, the brake system  10  can be provided with other braking functions such as anti-lock braking (ABS) and other slip control features to effectively brake the vehicle, as will be discussed below. 
     The brake system  10  generally includes a first block or brake pedal unit assembly, indicated by broken lines  12 , and a second block or hydraulic control unit, indicated by broken lines  14 . The various components of the brake system  10  are housed in the brake pedal unit assembly  12  and the hydraulic control unit  14 . As indicated, the brake pedal unit assembly  12  does not implement any valves. The brake pedal unit assembly  12  and the hydraulic control unit  14  may include one or more blocks or housings made from a solid material, such as aluminum, that has been drilled, machined, or otherwise formed to house the various components. Fluid conduits may also be formed in the housings to provide fluid passageways between the various components. The housings of the brake pedal unit assembly  12  and the hydraulic control unit  14  may be single structures or may be made of two or more parts assembled together. As schematically shown, the hydraulic control unit  14  is located remotely from the brake pedal unit assembly  12  with hydraulic lines hydraulically coupling the brake pedal unit assembly  12  and the hydraulic control unit  14 . 
     The brake pedal unit assembly  12  cooperatively acts with the hydraulic control unit  14  for actuating wheel brakes  16   a ,  16   b ,  16   c , and  16   d . The wheel brakes  16   a ,  16   b ,  16   c , and  16   d  can be any suitable wheel brake structure operated by the application of pressurized brake fluid (or pressure medium). The wheel brake  16   a ,  16   b ,  16   c , and  16   d  may include, for example, a brake caliper mounted on the vehicle to engage a frictional element (such as a brake disc) that rotates with a vehicle wheel to effect braking of the associated vehicle wheel. The wheel brakes  16   a ,  16   b ,  16   c , and  16   d  can be associated with any combination of front and rear wheels of the vehicle in which the brake system  10  is installed. For example, for a vertically split system, the wheel brakes  16   a  and  16   d  may be associated with the wheels on the same axle. For a diagonally split brake system, the wheel brakes  16   a  and  16   b  may be associated with the front wheel brakes. 
     The brake pedal unit assembly  12  includes a fluid reservoir  18  for storing and holding hydraulic fluid for the brake system  10 . The fluid within the reservoir  18  may be held generally at atmospheric pressure or can store the fluid at other pressures if so desired. The brake system  10  may include a fluid level sensor  19  for detecting the fluid level of the reservoir. The fluid level sensor  19  may be helpful in determining whether a leak has occurred in the system  10 . 
     The brake pedal control unit assembly  12  includes a brake pedal unit (BPU), indicated generally at  20 . It should be understood that the structural details of the components of the brake pedal unit  20  illustrate only one example of a brake pedal unit  20 . The brake pedal unit  20  could be configured differently having different components than that shown in  FIGS.  1 A- 1 B . 
     The brake pedal unit  20  includes a housing  24  having various bores formed in for slidably receiving various cylindrical pistons and other components therein. The housing  24  may be formed as a single unit or include two or more separately formed portions coupled together. The housing  24  generally includes a bore  32 . Bore  32  may have varying diameters as shown in  FIGS.  1 A- 1 B . The brake pedal unit  20  further includes an input piston (or primary piston)  34  and an output piston (or secondary piston)  40 . The input piston  34  and the output piston  40  may be slidably disposed in the bore  32 . 
     A brake pedal, indicated schematically at  42  in  FIGS.  1 A- 1 B , is coupled to a first end  44  of the input piston  34  via an input rod  45 . The input rod  45  can be coupled directly to the input piston  34  or can be indirectly connected through a coupler (not shown). In the rest position shown in  FIG.  1 A , an outer cylindrical surface  57  of the input piston  34  is engaged with a seal  58  and a lip seal  60  mounted in grooves formed in the housing  24 . The input piston  34  includes a central bore  62  formed through the second end  52 . The brake pedal unit  20  is in a “rest” position as shown in  FIG.  1 A . The conduit  66  is also in fluid communication with a first output pressure chamber  26  formed in the housing  24 . The conduit  66  is in fluid communication with a reservoir port  70  connected to the reservoir  18 . A filter (not shown) may be disposed in the port  70  or the conduit  66 . The conduit  66  can be formed by various bores, grooves and passageways formed in the housing  24 . 
     The pedal simulator  100  includes a chamber, a spring  130  and a piston  22 . It should be understood that that the various springs of the pedal simulator  100  may have any suitable spring coefficient or spring rate. The simulation chamber  63  may have brake fluid and may be in fluid communication with a conduit  47  which is in fluid communication with the simulation valve  74 . It is understood that region  68  is the dry region of the simulation chamber  63  because region  68  is the other side of the simulator piston). A filter (not shown) may be housed within the conduit  47 . Simulator test valve  82  is also provided in the hydraulic control unit  14  so that the two-way flow of the simulator valve  74  may be independently opened or closed without causing the brake fluid to flow back into the master cylinder pressure chamber and/or reservoir. 
     As discussed above, the brake pedal unit  20  includes the input and output pistons  34  and  40  that are disposed in bore  32  which is formed in the housing  24 . The input and output pistons  34  and  40  are generally coaxial with one another. A secondary output conduit  56  is formed in the housing  24  and is in fluid communication with the second output pressure chamber  28 . The secondary output conduit  56  may be extended via external piping or a hose connected to the housing  24 . A primary output conduit  66  is formed in the housing  24  and is in fluid communication with the first output pressure chamber  26 . The primary output conduit  66  may be extended via external piping or a hose connected to the housing  24 . As will be discussed in detail below, leftward movement of the input and output pistons  34  and  40 , as viewing  FIGS.  1 A- 1 B , provides pressurized fluid out through the secondary output conduit  56  and the primary output conduit  66 . A return spring  51  is housed in the first output pressure chamber  26  and biases the input piston  34  in the rightward direction. 
     The output piston  40  is slidably disposed in the bore  32 . A second output pressure chamber  28  is generally defined by the bore  32 , the output piston  40 , and the lip seal  54 . Leftward movement of the output piston  40  causes a buildup of pressure in the second output pressure chamber  28 . The second output pressure chamber  28  is in fluid communication with the secondary output conduit  56  such that pressurized fluid is selectively provided to the hydraulic control unit  14 . Second output pressure chamber  28  is in selective fluid communication with a conduit  64  which is in fluid communication with the reservoir  18 . 
     A first output pressure chamber  26  is generally defined by the bore  32 , the input piston  34 , the output piston  40 , the lip seal  60 , and the seal  53 . Although the various seals shown in the drawings are schematically represented as O-ring or lip seals, it should be understood that they can have any configuration. Leftward movement of the input piston  34  causes a buildup of pressure in the first output pressure chamber  26 . The first output pressure chamber  26  is in fluid communication with the primary output conduit  66  such that pressurized fluid is selectively provided to the hydraulic control unit  14 . 
     Referring again to  FIGS.  1 A- 1 B , the system  10  may further include a travel sensor  76  for producing a signal that is indicative of the length of travel of the input piston  34  which is indicative of the pedal travel. The system  10  may also include a switch  152  for producing a signal for actuation of a brake light and to provide a signal indicative of movement of the input piston  34 . The brake system  10  may further include sensors such as pressure transducers for monitoring the pressure in the conduit  56 . 
     The system  10  further includes a source of pressure in the form of a plunger assembly, indicated generally at  130 . As will be explained in detail below, the system  10  uses the plunger assembly  130  to provide a desired pressure level to the wheel brakes  16   a - d  during a normal boosted brake apply. Fluid from the wheel brakes  16   a - 16   d  may be returned to the plunger assembly  130  or diverted to the reservoir  18 . 
     The system  10  further includes a primary 3-way valve  38  and a secondary 3-way valve  36  (or referred to as switching valves or base brake valves). The three-way valves  36  and  38  may be solenoid actuated three-way valves. The three-way valves  36  and  38  are generally operable to up to three positions, as schematically shown in  FIGS.  1 A- 1 B . It is understood that three-way valve  38  is hydraulically moved to a “third” position during self-diagnostic testing as described herein. The secondary 3-way valve  36  has a port  36   a  in selective fluid communication with the secondary output conduit  56  which is in fluid communication with the second output pressure chamber  28 . A port  36   b  is in fluid communication with a boost conduit  160 . A port  36   c  is in fluid communication with a conduit  48  which is selectively in fluid communication with the wheel brakes  16   a  and  16   d . The primary 3-way valve  38  has a port  38   a  in selective fluid communication with the conduit  66  which is in fluid communication with the first output pressure chamber  26 . A port  38   b  is in fluid communication with the boost conduit  160 . A port  38   c  is in fluid communication with a conduit  72  which is selectively in fluid communication with the wheel brakes  16   b  and  16   c.    
     The system  10  further includes various valves (slip control valve arrangement) for permitting controlled braking operations, such as ABS, traction control, vehicle stability control, and regenerative braking blending. A first set of valves includes an apply valve  78  and a dump valve  80  in fluid communication with the conduit  48  for cooperatively supplying brake fluid received from the plunger assembly  130  to the wheel brake  16   d , and for cooperatively relieving pressurized brake fluid from the wheel brake  16   d  to the reservoir conduit  18  via the reservoir conduit  96 . A second set of valves include an apply valve  84  and a dump valve  86  in fluid communication with the conduit  48  for cooperatively supplying brake fluid received from the plunger assembly  130  to the wheel brake  16   a , and for cooperatively relieving pressurized brake fluid from the wheel brake  16   a  to the reservoir conduit  96 . A third set of valves include an apply valve  88  and a dump valve  90  in fluid communication with the conduit  72  for cooperatively supplying brake fluid received from the plunger assembly  130  to the wheel brake  16   c , and for cooperatively relieving pressurized brake fluid from the wheel brake  16   c  to the reservoir conduit  96 . A fourth set of valves include an apply valve  92  and a dump valve  94  in fluid communication with the conduit  72  for cooperatively supplying brake fluid received from plunger assembly  130  to the wheel brake  16   b , and for cooperatively relieving pressurized brake fluid from the wheel brake  16   b  to the reservoir conduit  96 . 
     As stated above, the system  10  includes a source of pressure in the form of the plunger assembly  130  to provide a desired pressure level to the wheel brakes  16   a - d . The system  10  further includes a venting valve  132  and a pumping valve  134  which cooperate with the plunger assembly  130  to provide boost pressure to the boost conduit  160  for actuation of the wheel brakes  16   a - 16   d . The venting valve  132  and the pumping valve  134  may be solenoid actuated valves movable between open positions and closed positions. In the closed position, the venting valve  132  and the pumping valve  134  may still permit flow in one direction as schematically shown as a check valve in  FIGS.  1 A- 1 B . The venting valve  132  is in fluid communication with a first output conduit  136  which is in fluid communication with the plunger assembly  130 . A second output conduit  138  is in fluid communication between the plunger assembly  130  and the boost conduit  160 . 
     The plunger assembly  130  includes a housing  104  having a multi-stepped bore  118  formed therein. A piston  110  is slidably disposed with the bore  118 . The piston  110  includes an enlarged end portion  112  connected to a smaller diameter central portion  114 . The piston  110  has a second end  116  connected to a ball screw mechanism, indicated generally at  120 . The ball screw mechanism  120  is provided to impart translational or linear motion of the piston  110  along an axis defined by the bore  118  in both a forward direction (leftward as viewing  FIGS.  1 A- 1 B ), and a rearward direction (rear/right-ward as viewing  FIGS.  1 A- 1 B ) within the bore  118  of the housing  104 . In the embodiment shown, the ball screw mechanism  120  includes a motor  122  rotatably driving a screw shaft  124 . The motor  122  may include a sensor  126  for detecting the rotational position of the motor  122  and/or ball screw mechanism  120  which is indicative of the position of the piston  110 . The second end  116  of the piston  110  includes a threaded bore and functions as a driven nut of the ball screw mechanism  120 . The ball screw mechanism  120  includes a plurality of balls that are retained within helical raceways formed in the screw shaft  124  and the threaded bore of the piston  110  to reduce friction. Although a ball screw mechanism  120  is shown and described with respect to the plunger assembly  130 , it should be understood that other suitable mechanical linear actuators may be used for imparting movement of the piston  110 . It should also be understood that although the piston  110  functions as the nut of the ball screw mechanism  120 , the piston  110  could be configured to function as a screw shaft of the ball screw mechanism  120 . Of course, under this circumstance, the ball nut would rotate and the screw shaft would translate so as to move the plunger  110  relative to the multi-stepped bore  118  as the ball nut is rotated via the motor  122 . 
     As will be discussed in detail below, the plunger assembly  130  can provide boosted pressure to the boost conduit  160  when actuated in both the forward and rearward directions. The plunger assembly  130  includes a seal  140  mounted on the enlarged end portion  112  of the piston  110 . The seal  140  slidably engages with the inner cylindrical surface of the bore  118  as the piston  110  moves within the bore  118 . A pair of seals  142  and  144  is mounted in grooves formed in the bore  118 . The seals  142  and  144  slidably engage with the outer cylindrical surface of the piston  110 . A first pressure chamber  150  is generally defined by the bore  118 , the enlarged end portion  112  of the piston  110 , and the seal  140 . A second pressure chamber  151 , located generally behind the enlarged end portion  112  of the piston  110 , is generally defined by the bore  118 , the seals  142  and  140 , and the piston  110 . The seals  140 ,  142 , and  144  can have any suitable seal structure. In one embodiment, the seal  140  is a quad ring seal. Although a lip seal may also be suitable for the seal  140 , a lip seal is more generally more compliant and requires more volume displacement for a given pressure differential. This may result in a small boost pressure reduction when the piston  110  travels in the rearward direction during a pumping mode. The lip seal or seal  140  may be provided in the form of an o-ring energized PTFE seal because this component can tolerate big extrusion gaps. 
     As shown in  FIGS.  1 A- 1 B , the hydraulic control unit  14  also includes a simulator test valve  82  and a simulator valve  74  which may be mounted proximate to the brake simulator. The simulator test valve  82  is generally not used during a normal boosted brake apply or even for a manual push-through mode. The simulator test valve may be energized or de-energized during various testing modes to determine the correct operation of the brake system  10 . The simulator test valve  82  may be energized to a “closed position” (uni-directional flow within the valve  82 ) to prevent venting pressure medium away from the primary output chamber via the conduit  66  such that a pressure build up in conduit  160  can be used to monitor fluid (or pressure medium) flow to determine if leaks may be occurring through seals of various components of the brake system  10 . 
     As schematically shown in  FIGS.  1 A- 1 B , the simulation valve  74  may be a solenoid actuated valve. The simulation valve  74  includes a first port  75  and a second port  77 . The first port  75  is in fluid communication with the conduit  47  and the simulation chamber  63  of the simulator  100 . The second port  77  is in fluid communication with the reservoir  18  via the conduits  66  and  70 . The simulation test valve  82  is movable between a first open (de-energized) position allowing the flow of fluid from the simulation chamber  63  to the first output pressure chamber  26 , and a second closed position blocking flow of fluid or pressure medium between the first output pressure chamber  26  and the simulation chamber  63  only in direction from simulator to master cylinder. The simulation valve  74  is in the first closed position or normally closed position when not actuated (not energized) such that fluid is prevented from flowing into the simulation chamber  63  through conduit  160 . 
     The simulator valve  74  may be energized together with the apply valves  78 ,  84 ,  92 ,  88  and while the simulator test valve  82  is energized (so as to close/block pressure medium from flowing from conduit  160  to conduit  66 ) and the simulator valve  74  is energized so as to open simulator valve  74  (to increase fluid flow through simulator valve  74 ) so that the dual acting plunger  110  can be used to fill the pedal simulator  100  via conduit  160  with the pressure medium so that the system may perform self-diagnostic tests as later described herein. Under this circumstance, the secondary 3-way valve and the pumping valve are also energized. 
     Example, non-limiting diagnostic operations of the aforementioned system may include: (1) a diagnostic test for a leak in the Simulator Valve; (2) an optional diagnostic test for a leak in the Pedal Simulator; and/or (3) a diagnostic test for a leak in the primary and/or secondary three-way valves. 
     In order to perform a diagnostic test for a leak in the simulator valve  74  in the example system of  FIG.  1 A , the system  10  may perform the following steps: (1) energize the secondary three-way valve  36 , the pumping valve  134 , the simulator test valve  82 , and the apply valves  78 ,  84 ,  88 ,  92  in order to maintain brake fluid from the plunger assembly  130  within the boost circuit  160 ; (2) Apply the dual acting plunger  110  in the plunger assembly  130  to a predetermined pressure level such as 30 Bar in the boost circuit  160  wherein the boost pressure sensor  148  is used to determine the pressure level in the boost circuit  160 ; (3) Hold the plunger  110  in position within the plunger assembly and if the pressure in the boost circuit  160  deteriorates at or more than a pre-determined rate (ex: more than 20 Bar in 100 msec), then identify a leak in the simulator valve  74  via a signal from the ECU  106  to a vehicle user interface  108 . Alternatively, if the plunger  110  has to travel more than a pre-determined distance (ex: 4 mm) in order to achieve a pressure of 30 Bar in the boost circuit  160  (at step 2), then identify a leak in the simulator valve  74  via a signal  146  from the ECU  106  to a vehicle user interface  108 . 
     Following the test for a leak in the simulator valve  74  and with reference to  FIG.  1 B , the system  10  may perform another diagnostic test for a leak in the pedal simulator  100  by performing the following steps: (1) de-energize the simulator test valve  82  to release pressure; (2) energize the pedal simulator valve  74  and then energize the simulator test valve  82 ; (3) apply the dual acting plunger  110  (while energizing the secondary three-way valve  36 , the pumping valve  134 , the simulator test valve  82 , and the apply valves  78 ,  84 ,  88 ,  92 ) to achieve a predetermined pressure in the pedal simulator  100 ; (4) hold the dual acting plunger  110  in position once the predetermined pressure has been achieved in the pedal simulator and if the pressure in the in the pedal simulator  100  deteriorates at a pre-determined rate (ex: more than 3 Bar in 100 msec), then identify a leak in the pedal simulator  100  via a signal  146  from the ECU  106  to a vehicle user interface  108 . Alternatively, if the dual acting plunger  110  in the plunger assembly  130  has to travel more than a pre-determined distance (ex: 5 mm to 20 mm) in order to achieve a pressure of 10 Bar in the pedal simulator (at step 3) then identify a leak in the pedal simulator  100  via a signal  146  from the ECU  106  to a vehicle user interface  108 . 
     Following the test of the pedal simulator  100  for leaks, the above-referenced system  10  may also test for a leak in the three-way valves  36 ,  38 . However, in the event a leak has been detected in the pedal simulator  100 , any boost pressure from the previous test must be released before testing for a leak in the three-way valves  36 ,  38 . Accordingly, such boost pressure may be released from the boost circuit  160  by de-energizing the simulator test valve  82  and moving the plunger  110  to the home position. In contrast, if a leak has not been detected in the pedal simulator  100 , then the aforementioned boost pressure release is not required. 
     Once the boost pressure has been reduced in the boost circuit  160 , the method for testing for a leak in the three way valves includes the following steps: (1) fill the pedal simulator  100  using the same method used as was used in the aforementioned simulator leak detection process/method; (2) De-energize the secondary 3-way valve and then retract the dual acting plunger to the home position to further drop the pressure in the boost circuit to zero while also monitoring the master cylinder secondary pressure sensor  98  (which is indicative of pressure at the output of the fluid separator  156  during this test method); (3) Determine whether there is a pressure reduction at the secondary master cylinder pressure sensor  98 . If the pressure deteriorates at a pre-determined rate (more than 7 bar in 100 msec), then then identify a leak in the three-way valves  36 ,  38  via a signal from the ECU to a vehicle user interface. However, if the pressure drop does not exceed the predetermined threshold, then the HCU determines that there is not a leak in either of the three-way valves  36 ,  38 . 
     Referring now to  FIGS.  2 A- 2 B , an orifice (see element  30 ) may be implemented at the master cylinder  50  (in the conduit  67  between the venting port  154  of the first output pressure chamber  26  and the reservoir  18 ) instead of using a simulator test valve (see element  82  in  FIGS.  1 A and  1 B ) in the brake module (see element  14  in  FIGS.  1 A- 1 B ). The system  128  shown in  FIGS.  2 A- 2 B  may perform the following steps to determine if there is a leak in the simulator valve  74  (shown in  FIGS.  2 A- 2 B ): (1) provide a simulator  100  (see  FIG.  2 A ) which is only partially full of pressure medium and a de-energized simulator valve  74 ; (note: when de-energized, the simulator valve  74  only allows fluid to flow in one direction—from the simulator  100  towards conduit  66 , but when energized, the simulator valve  74  may, but not necessarily, permit fluid to flow in both directions) (2) Energize the pumping valve  134 , the secondary three way valve  36 , and the apply valves  78 ,  84 ,  92 ,  88  so that all apply valves  78 ,  84 ,  92 ,  88  are closed (where pressure medium is only able to flow from the dump valves  80 ,  86 ,  94 ,  90  towards the corresponding three way valve  36 ,  38 ); (3) apply and retract the dual acting plunger (or piston)  110  in the plunger assembly  130  so that a predetermined pressure is achieved at the master cylinder secondary pressure sensor  98 ; (4) Hold the plunger  110  in position, but maintain the replenishing check valve  102  in a closed position this automatically happens when the plunger stops moving (to prevent pressure medium from flowing from the plunger assembly  130  to reservoir  18 ) while energizing the simulator valve  74  (to enhance medium flow from the simulator chamber  63  towards the orifice  30 ); (5) Measure master cylinder secondary pressure decay to obtain the measured master cylinder secondary pressure decay; (6) Compare measured master cylinder secondary pressure decay to a predetermined master cylinder secondary pressure decay value. (The predetermined master cylinder secondary pressure decay value may be taken from a master cylinder secondary pressure decay rate obtained when the system was new and the application of the master cylinder was used to verify that the simulator valve did not leak); (7) If the measured master cylinder secondary pressure decay does not match the predetermined master cylinder secondary pressure decay, then then identify a leak in the simulator valve via a signal from the ECU to a vehicle user interface. (8) De-energize all the valves. It is understood that the system may optionally indicate that there is no leak in the simulator valve via a signal from the ECU to a vehicle user interface if the measured master cylinder secondary pressure decay does match the predetermined master cylinder secondary pressure decay. 
     In step 1 of the aforementioned method, the simulator  100  may be partially filled with pressure medium by energizing the pumping valve  134 , the secondary three way valve  36 , and the apply valves  78 ,  84 ,  92 ,  88  and then applying and retracting the plunger in the plunger assembly (while keeping the simulator valve open or energized) until the pressure in the second output pressure chamber of the master cylinder reads about 1.5 bar and then de-energize the simulator valve while completing the stroke of the plunger in the plunger assembly. 
     While various example, non-limiting embodiments have been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the disclosure in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the disclosure as set forth in the appended claims and the legal equivalents thereof.