Abstract:
An in-line brake system with a hydraulic booster includes a master cylinder defining a cylinder bore, a master cylinder piston located within the cylinder bore, a transfer piston located within the cylinder bore rearwardly of the master cylinder piston, a transfer piston actuator with a first seat, a sealing member aligned with the first seat, a sealing member spring operably connected to the sealing member and the transfer piston, an input rod aligned with the first seat, a return spring operably connected to the input rod and the transfer piston actuator, a sleeve actuator located within the cylinder bore rearwardly of the master cylinder, and a sleeve spring configured to bias the sleeve actuator away from the master cylinder piston, wherein the sleeve actuator is configured to bias the transfer piston toward the master cylinder piston in response to an applied boost pressure.

Description:
FIELD 
     The invention relates to braking systems, and in particular to a braking system with a pedal feel simulator. 
     BACKGROUND 
     Significant progress has been made in vehicular braking systems in recent years. Among these developments are different braking strategies such as anti-lock braking systems (ABS) and regenerative braking systems. The latter is used in electric and hybrid-electric vehicles. These braking strategies are interchangeably blended in order to brake a vehicle. Typically, a vehicle&#39;s brake pedal is mechanically decoupled from downstream braking circuits. A control valve typically regulates boost pressure from an accumulator to provide a regulated boost pressure to the downstream braking circuits. Since the brake pedal is mechanically decoupled, a brake pedal feel simulator is typically included in a braking system in order to provide a feedback to the operator of the vehicle. 
     In the event of a failure of the hydraulic system and/or the electrical regenerative system, it is necessary for the braking system to switch modes of operation so that the brake pedal is mechanically coupled to the downstream brake circuits. In such a failure mode, the force applied to the brake pedal is transferred to the downstream brake circuits to generate the necessary braking force to halt a vehicle. 
     There is a need to provide an improved braking system that is operable in a normal mode in which the brake pedal is mechanically decoupled from the downstream braking circuits and also operable in a failure mode in which the brake pedal is at least mechanically coupled to one downstream braking circuit. 
     SUMMARY 
     According to one embodiment of the present disclosure, there is provided an in-line brake system with a hydraulic booster. The in-line brake system with a hydraulic booster includes a master cylinder defining a cylinder bore, a master cylinder piston located within the cylinder bore, a transfer piston located within the cylinder bore rearwardly of the master cylinder piston, a transfer piston actuator with a first seat, a sealing member aligned with the first seat, a sealing member spring operably connected to the sealing member and the transfer piston, an input rod aligned with the first seat, a return spring operably connected to the input rod and the transfer piston actuator, a sleeve actuator located within the cylinder bore rearwardly of the master cylinder, and a sleeve spring configured to bias the sleeve actuator away from the master cylinder piston, wherein the sleeve actuator is configured to bias the transfer piston toward the master cylinder piston in response to an applied boost pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a partial cross sectional view of a braking system including a booster portion which has a transfer piston assembly, an input rod assembly, and a sleeve assembly, and a master cylinder portion which has a master cylinder piston assembly; 
         FIG. 2  depicts a partial cross sectional view of the master cylinder piston assembly of  FIG. 1 ; 
         FIG. 3  depicts a partial cross sectional view of the transfer piston assembly of  FIG. 1 , including an actuator shown in phantom; 
         FIG. 4  depicts a partial cross sectional view of the input rod assembly of  FIG. 1  including a seal member actuator; 
         FIG. 5  depicts a partial cross sectional view of the sleeve assembly of  FIG. 1 ; 
         FIG. 6  depicts the braking system of  FIG. 1  in an initial actuation position in a normal operating mode, wherein a seal member actuator has moved leftward and has caused leftward movement of a seal member off of a first seat to thereby provide regulated high pressure fluid to the sleeve assembly; 
         FIG. 7  depicts the braking system of  FIG. 1  in an actuated position in the normal mode, wherein the seal member actuator has moved further leftward as compared to  FIG. 6 , thereby increasing pressure of the regulated high pressure fluid within the sleeve assembly; and 
         FIG. 8  depicts the braking system of  FIG. 1  in an initial actuation position in a failure operating mode, wherein the high pressure fluid is inoperative and the seal member cannot regulate pressure within the sleeve assembly. 
     
    
    
     DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one of ordinary skill in the art to which this invention pertains. 
     Referring to  FIG. 1 , a partial cross sectional view of a braking system  100  is depicted. The braking system  100  includes a booster portion  102  and a master cylinder portion  104 . The booster portion  102  and the master cylinder portion  104  are enclosed by a master cylinder  106  which extends over the boost portion  102  and the master cylinder portion  104 . The master cylinder  106  has a bore  107  which extends over substantially the entire length of the master cylinder  106 . 
     Within the bore  107 , the braking system  100  includes a master cylinder piston assembly  108 , a transfer piston assembly  110 , and an input rod assembly  112 . An input shaft  114  is coupled to a brake pedal (not shown) at one end and fixedly coupled to the input rod assembly  112  at another end. The input shaft  114  interfaces with the input rod assembly  112  in a press fit manner or other known manners in which the input shaft  114  can swivel with respect to the input rod assembly  112 . The input shaft  114  is configured to transfer forces applied to the brake pedal (not shown) to the input rod assembly  112 . The input rod assembly  112  is biased away from the transfer piston assembly  110  by an input rod spring  136 . The input rod spring may also be referred to as the return spring  136 . Also depicted in  FIG. 1  is a sleeve assembly  116 . 
     The master cylinder piston assembly  108  is sealingly positioned within the bore  107  of the master cylinder  106  and defines a master cylinder chamber  109  between the bore  107  and the master cylinder piston assembly  108 . The master cylinder chamber  109  is in fluid communication with a first downstream braking circuit (not shown). The master cylinder chamber  109  is also in selective communication with a reservoir (not shown), as described in greater detail below. 
     The sleeve assembly  116  and the bore  107  define a first sleeve chamber  118  which is in fluid communication with a fluid channel  121 . The fluid channel  121  is also in fluid communication with a second downstream braking circuit (not shown) via an outlet  122 . Specifically, an accumulator (i.e., source of high pressure fluid, not shown) is in selective fluid communication with a fluid channel  121 , as described in greater detail below. The fluid channel  121  is in fluid communication with a chamber  123  which is in fluid communication with outlets  125 . The outlets  125  are in fluid communication with a radial fluid channel  127 ′ which is fluidly coupled to an axial fluid channel  129 . The axial fluid channel  129  is coupled to a radial fluid channel  127 ″ which is in fluid communication with the first sleeve chamber  118 . The first sleeve chamber  118  is in fluid communication with the outlet  122 , which as described above, is in fluid communication with the second braking circuit (not shown). 
     An inlet  124  provides fluid communication between the reservoir (not shown) and a second sleeve chamber  126  which is defined by the sleeve assembly  116  and the bore  107 . Therefore, the second sleeve chamber  126  is in continuous fluid communication with the reservoir (not shown). 
     The bore  107 , the master cylinder piston assembly  108 , and the transfer piston assembly  110  define a boost chamber  128  which is in fluid communication with the axial fluid channel  129  via a radial fluid path  127 ′″ and via another fluid path  131 . Since the internal fluid chamber is in fluid communication with the first sleeve chamber  118  and with the boost chamber  128 , the boost chamber  128  is also in fluid communication with the first sleeve chamber  118 . The transfer piston assembly  110  is also sealingly coupled to the sleeve assembly  116 , as will be discussed in greater detail below. 
     The master cylinder piston assembly  108  and the sleeve assembly  116  are biased away from each other by a sleeve spring  130 . The sleeve spring  130  is positioned within the boost chamber  128  and is configured to compress as distance between the master cylinder piston assembly  108  and the sleeve assembly  116  decreases. 
     A sealing member  132  is disposed between the transfer piston assembly  110  and the input rod assembly  112 . The sealing member  132  is substantially in the form of a ball. As will be described more fully below, the sealing member  132  is configured to seal the fluid channel  121  from the accumulator (not shown) when the brake pedal (not shown) is in a released position (i.e., in an unactuated position). Furthermore, as discussed below, the sealing member  132  is configured to modulate fluid pressure within the fluid channel  121  in response to a force applied to the brake pedal (not shown). 
     As depicted in  FIG. 1 , there is a gap  134  between the master cylinder piston assembly  108  and the transfer piston assembly  110 . A spring  135  disposed between the master cylinder piston assembly  108  and the transfer piston assembly  110  is configured to bias the master cylinder piston assembly  108  and the transfer piston assembly  110  away from each other. 
     Referring to  FIG. 2 , a partial cross sectional view of the master cylinder piston assembly  108  is depicted. The master cylinder piston assembly  108  includes a body portion  152 , a master cylinder piston bracket  154 , a master cylinder piston spring  156 , and a master cylinder end bracket  158 . The body portion  152  is fixedly coupled to the master cylinder piston bracket  154 , e.g., in a press fit manner. 
     The master cylinder piston spring  156  biases the master cylinder piston bracket  154  away from the master cylinder end bracket  158 . The body portion  152  is coupled to a master cylinder valve assembly, e.g., a poppet valve (not shown), via a piston rod  159  which centrally extends through the master cylinder piston bracket  154 . The piston rod  159  is slidably disposed within a central cavity  160  of the body portion  152 . 
     The body portion  152  further includes a rear portion  162 . The rear portion  162  includes a transfer piston interface  164  which provides a sealing surface for the transfer piston assembly  110 , as further described below. The transfer piston interface  164  terminates at an end surface  166  which is also configured to interface with the transfer piston assembly  110 . In addition, the seal  168  provides a sealed interface between the body portion  152  and the bore  107  of the master cylinder  106 . 
     Referring to  FIG. 3 , a partial cross sectional view of the transfer piston assembly  110  is depicted. The transfer piston assembly  110  includes a body portion  201 , and a nose portion  202 . The nose portion  202  is an extension of the body portion  201  and is configured to be positioned within the rear portion  162  of the master cylinder piston assembly  108 . Specifically, the transfer piston interface  164  encloses a portion of the nose  202  (see also  FIGS. 1 and 2 ). 
     The transfer piston assembly  110  also includes an inlet  206  which is fluidly coupled to the accumulator (not shown). The inlet  206  provides a fluid channel  207  between the accumulator (not shown) and the sealing member  132  (shown in phantom). 
     The transfer piston assembly  110  also includes a control valve spring  208 , an inlet portion  209 , and an inlet member  210 . The control valve spring  208 , is hereinafter also referred to as the sealing member spring. The control valve spring  208  biases the inlet member  210  away from the inlet portion  209 . As depicted in  FIG. 3 , the inlet member  210  is slidably disposed within the fluid channel  207 , and is configured to move therein. Due to the control valve spring  208 , the inlet member  210  is positioned to make contact with the sealing member  132  (shown in phantom, with reference to  FIG. 3 ). The inlet member  210  terminates at a contact ring  214  which is configured to interface with the sealing member  132 . The contact ring  214  may be integrally formed with the inlet member  210  or may be a member that can be separated and replaced due to wear. 
     As depicted in  FIG. 3 , the sealing member  132  (shown in phantom) contacts a transfer piston actuator  232 . The transfer piston actuator  232  is integrally formed with the body portion  201 . The transfer piston actuator includes a first seat  236  and a second seat  238 . The first seat  236  is configured to seal against the sealing member  132  and thereby seal the fluid channel  121 , as described further below. The first seat  236  may be an integrally formed part of the body portion  232  or may be a separate member configured to be assembled on to the body portion  232  and also to be removed for service due to wear. The second seat  238  is configured to contact a seat  261  of the input rod assembly  112 , as described further below. The transfer piston actuator  232  also includes a rear portion  220 . The rear portion  220  includes a seat  234  which is configured to provide a seat for the spring  136  (shown in phantom). The spring  136  is positioned between the seat  234  and the input rod assembly  112 . The spring  136  is configured to bias the transfer piston assembly  110  and the input rod assembly  112  away from each other during actuation of the braking system  100  as discussed below and also with reference to  FIG. 4 . 
     Referring to  FIG. 4 , a partial cross sectional view of the input rod assembly  112  is depicted. The input rod assembly  112  includes a pin  252 , a collar  254 , an input rod  256 , and a body portion  258 . The pin  252  includes a bore  253  which is aligned with the fluid channel  121 . The pin  252  defines an end  257  which is configured to make contact with the sealing member  132 , as further described below, and in doing do establish fluid communication between the accumulator (not shown), the surroundings of the sealing member  132 , the bore  253 , and the fluid channel  121  (see  FIG. 1 ). The end  257  may include a castelating feature that enables the end  257  to contact the sealing member  132  while providing an unobstructed fluid path to the bore  253 . 
     The collar  254  includes a seat  261  which is configured to come in contact with the second seat  238  of the transfer piston actuator  232  (see  FIG. 3 ) under certain operational conditions as described further below. 
     The collar  254  may be integrally formed with the input rod  256 . The input rod  256  includes a bore which defines the fluid channel  121 . The input rod  256  also defines a step  264 . The step  264  is configured to receive one end of the input rod spring  136 , while as discussed above, the seat  234  of the rear portion  220  of the transfer piston actuator  232  is configured to receive the other end (see  FIG. 3 ). The input rod  256  is slidably coupled to the body portion  258  (i.e., the input rod  256  can slide with respect to the body portion  258 ). 
     The body portion  258  includes seal housings for receiving seals  266  and  268 . The seals  266  and  268  seal the input rod assembly  112  against the bore  107  of the master cylinder  106 . The body portion  258  also includes a seal housing for receiving a seal  270 . The input rod  156  also includes a seal housing for receiving a seal  272 . The seals  270  and  272  seal the input rod against the body portion  258 . 
     The interface between the input rod  256  and the body portion  258  defines the chamber  123 , which as discussed above is in fluid communication with the fluid channel  121  and the outlet  125 . 
     The input rod  245  further includes a rear portion  260  which defines a cavity  262 . The cavity  262  is configured to receive the input shaft  114 , as described above, in a press fit manner in which the input shaft  114  once inside the cavity  262  is allowed to swivel with respect to input rod  256 . 
     Referring to  FIG. 5 , a partial cross sectional view of the sleeve assembly  116  is provided. The sleeve assembly  116  includes a sleeve  302  and seal housings for receiving seals  304  and  305 . The sleeve  302  includes a radial notch  308  for receiving a retaining ring  306 . The retaining ring  306  defines an inner diameter that is larger than the outer diameter of the input rod spring  136  and the outer diameter of the input rod  256  (see  FIGS. 3 and 4 ). Therefore, the input rod  256  can slide through the retaining ring  306 , while compressing the input rod spring  136  without interference with the inner diameter of the retaining ring  306 . The seal housing for the seal  306  defines an active face  310  for communicating with fluid within the first sleeve chamber  118 . 
     The operation of the braking system  100  is described herein with initial reference to  FIG. 1 . Two modes of operation are described. In general, during a normal mode of operation, the accumulator (not shown) supplies high pressure fluid to the braking system  100  in order to provide a hydraulic boost function, known in the art. In a failure mode, however, the accumulator is unable to supply the high pressure fluid, and thereby no boost function is provided. 
     Referring back to  FIG. 1 , general operational conditions with respect to both normal and failure modes are described. The input shaft  114  is coupled to the brake pedal (not shown). The brake pedal (not shown) in a released position results in the position of the braking system  100  that is depicted in  FIG. 1 , which is hereinafter referred to as the “rest” position. 
     Starting with the master cylinder piston assembly  108  and its fluid coupling to the first downstream braking circuit (not shown) and the reservoir (not shown), the body portion  152  is biased rightward. This is because, the body portion  152  is fixedly coupled to the master cylinder piston bracket  154 , and the master cylinder piston spring  156  biases the master cylinder piston bracket  154  away from the master cylinder end bracket  158  (see  FIG. 2 ). 
     The piston rod  159  is coupled to the master cylinder valve assembly (not shown) on the left side of the piston rod and coupled to the master cylinder piston bracket  154  on the right side. The master cylinder piston bracket  154  limits leftward travel of the piston rod  159  beyond the position that is depicted in  FIG. 1 . 
     The transfer piston assembly  110  is disposed to the right (rearwardly) of the master cylinder piston assembly  108 . The control valve spring  208  biases the inlet member  210  away from the inlet portion  209  and into contact with the sealing member  132 . (see  FIG. 3 ). Therefore, the sealing member  132  is biased rightward and is thereby firmly seated on the first seat  236  of the transfer piston actuator  232 . 
     With the first seat  236  firmly seated on the sealing member  132  the fluid channel  121  is isolated from fluid communication with the accumulator (not shown). 
     In  FIG. 1 , the end  257  of the pin  252  is positioned proximate to the sealing member  132 . The end  257  is depicted to be substantially out of contact with the sealing member  132  only to make it clear to the reader that the pin  252  is exerting minimal or zero force on the sealing member  132 , in the rest position. Also, in the rest position, the input rod spring  136  which is disposed between the step  264  of the input rod  256  and the seat  234  of the transfer piston actuator  232  is minimally compressed, or simply not compressed at all. 
     The sleeve assembly  116  is biased away from the master cylinder piston assembly  108  by the sleeve spring  130 . As a result the sleeve assembly  116  is in contact with the body portion  258  of the input rod assembly  112 . 
     In the rest position, the master cylinder chamber  109  is in fluid communication with the reservoir (not shown) via the master cylinder valve assembly (not shown). The master cylinder chamber  109  is also in fluid communication with the first downstream circuit (not shown). Therefore, pressure within the master cylinder chamber  109  and the first downstream braking circuit (not shown) is negligible (i.e., the same as the fluid pressure of the reservoir, not shown). 
     The accumulator (not shown) provides high pressure fluid to the transfer piston assembly  110  at the inlet  206  (see  FIG. 3 ). On the inlet member  210  side of the sealing member  132  the pressure is the same as the accumulator pressure. Since the sealing member  132 , however, is firmly seated on the first seat  236  (see  FIG. 3 ), the bore  253  and the fluid channel  121  are fluidly isolated from the accumulator (not shown). Therefore, in the rest position, pressure within the bore  253 , the fluid channel  121 , the chamber  123 , the outlet  125 , the radial fluid channels  127 ′,  127 ″, and  127 ′″, and the second downstream braking circuit (not shown) are negligible. Also, since the boost chamber  128  is in fluid communication with the fluid channel  121 , fluid pressure within the boost chamber is also negligible. 
     Referring to  FIG. 6 , an initial actuation position of the braking system  100  in the normal mode is depicted. When an operator of the vehicle applies a force to the brake pedal (not shown), the input shaft  114  moves leftward. Leftward movement of the input shaft  114  moves the input rod  256  of the input rod assembly  112  leftward in the direction of an arrow  300 . Since the input rod spring  136  is disposed between the step  264  of the input rod  256  (see  FIG. 4 ) and the seat  234  of the transfer piston actuator  232  (see  FIGS. 3 and 4 ), leftward movement of the input rod  256  biases the transfer piston actuator  232  leftward by the force of the input rod spring  136 . The pin  252  which is disposed between the collar  254  and the sealing member  132  comes in contact with the sealing member  132 . The pin  252  applies a leftward force to the sealing member  132  which is transferred to the control valve spring  208 . The spring constant of the control valve spring  208 , the spring  135 , and the master cylinder piston spring  156  are chosen so that the control valve spring  208  begins to compress. The compression of the control valve spring  208  allows the end  257  of the pin  252  to unseat the sealing member  132  from the first seat  236 . 
     The unseating of the sealing member  132  from the first seat  236  provides fluid communication between the accumulator (not shown) and the fluid channel  121  via the bore  253  (see also  FIG. 4 ) by allowing fluid to rush by the first seat  236 . The pressure within the fluid channel  121  is proportional to the force applied by the operator to the brake pedal (not shown). Therefore, the sealing member  132  and the first seat  236  regulate the pressure within the fluid channel  121 . 
     The rushing fluid around the first seat  236  moves into the fluid channel  121 . Since the fluid channel  121  is in fluid communication with the first sleeve chamber  118 , pressure rises therein. With the second sleeve chamber being coupled to the reservoir (not shown), pressure within the first sleeve chamber  118  rises above pressure within the second sleeve chamber  126 . The pressure differential between these chambers  118  and  126  causes the sleeve assembly  116  to move leftward in the direction of the arrow  300  against a biasing force provided by the sleeve spring  130  positioned between the sleeve  302  and the master cylinder piston assembly  108 . In response to the leftward movement of the sleeve assembly  106 , the retaining ring  306  also moves leftward, thereby contacting the rear portion  220  of the transfer piston assembly  110 , as depicted in  FIG. 6 . 
     Furthermore, since the fluid channel  121  is also fluidly coupled to the boost chamber  128 , pressure rises therein as well. Fluid pressure within the boost chamber  128  actuates the master cylinder piston assembly  108  and moves it leftward. Leftward movement of the master cylinder piston assembly  108  closes the master cylinder valve assembly (not shown) and thereby isolates the master cylinder chamber  109  from the reservoir (not shown). Therefore, fluid pressure within the master cylinder chamber  109  begins to rise as the master cylinder piston assembly  108  moves leftward. 
     With the first and second downstream braking circuits coupled to the master cylinder chamber  109  and the fluid channel  121 , respectively, pressures within these circuits begin to rise, thereby providing the desired braking function. The reader should appreciate that the spring constant of the spring  135  (K 135 ) is higher than the spring constant of the input rod spring  136  (K 136 ) which is higher than the spring constant of the control valve spring  208  (K 208 ). That is, K 135 &gt;K 136 &gt;K 208 . 
     An increase in the force applied to the brake pedal (not shown) by the operator generates further leftward movement of the input shaft  114 . Referring to  FIG. 7 , a subsequent actuated position of the braking system  100  in the normal mode of operation is depicted. In the subsequent actuated position, the input rod  256  has further moved leftward in the direction of the arrow  300 , which has caused the pin  252  to further move leftward. The leftward movement of the pin  252  has further moved the sealing member  132 , thereby further unseating the sealing member  132  from the first seat  236  as compared to the position of the sealing member  132  of  FIG. 6 . As a result, pressure within the fluid channel  121  increases, which increases pressures within the first sleeve chamber  118  and the boost chamber  128 . The reader should appreciate that the relative positions of the first seat  236  and the sealing member  132  are depicted in an exaggerated manner for clarity. In reality, a very small space is provided between the sealing member  132  and the first seat  236 . 
     With the increased pressure in the boost chamber  128 , the master cylinder piston assembly  108  moves further leftward causing pressure within the master cylinder chamber  109  to further increase. Notably, the piston rod  159  moves within the central cavity  160 . 
     Further leftward movement of the master cylinder piston assembly  108  generates increased pressure in the first downstream braking circuit (not shown). Similarly, increased pressure within the fluid channel  121  results in increased pressure in the second downstream braking circuit (not shown). 
     The leftward movement of the master cylinder piston assembly  108  has the potential of increasing the gap  134 , however, as described below the braking system  100  is configured to maintain the gap  134 . 
     The increased pressure within the fluid channel  121  increases pressure within the first sleeve chamber  118 . As discussed above, pressure within the second sleeve chamber  126  remains at the pressure of the reservoir (not shown). Therefore, the pressure differential between the first and second sleeve chambers  118  and  126  increases. Because of the increased pressure differential, the sleeve assembly  116  moves further leftward, which moves the retaining ring  306  further leftward. Since the retaining ring  306  is already in contact with the rear portion  220  of the transfer piston assembly  110 , as depicted in  FIG. 6 , the further leftward movement of the retaining ring  306  causes further leftward movement of the transfer piston assembly  110 . 
     The active face  310  of the sleeve assembly  116  is so dimensioned that regulated pressure of fluid within the fluid channel  121  (and thereby within the first sleeve chamber  118 ) acting on it provides sufficient leftward force (and thereby leftward movement) to overcome the biasing force of the sleeve spring  130 . By choosing the appropriate spring constant of the spring  130  and dimensions of the active face  310 , the leftward movement of the sleeve assembly  116 , and therefore the transfer piston assembly  110 , can be matched to the leftward movement of the master cylinder assembly  108 , to thereby maintain the gap  134 . 
     Since the brake pedal (not shown) is mechanically decoupled from the downstream braking circuits (not shown), the input rod spring  136  and the control valve spring  208  provide a feedback to the operator in the normal mode through the sealing member  132 , the pin  252 , the input rod  256 , and the input shaft  114 . 
     In the failure mode, the accumulator (not shown) is unable to supply fluid at a pressure necessary to provide the boost function. In reference to  FIG. 8 , an initial actuated position of the braking assembly  100  in the failure mode is depicted. In the failure mode, the braking system  100  operates similar to the normal mode, with few differences. Since the accumulator (not shown) is inoperative, the pressure within the fluid channel  121  remains low. As a result, there is no braking generated in the second downstream braking circuit (not shown). Pressures within the boost chamber  128  and the first sleeve chamber  118  remain low, thereby affecting no leftward forces for moving the master cylinder piston assembly  108  and the sleeve assembly  116 . 
     In the failure mode, however, braking is generated in the first downstream braking circuit (not shown), as described below. The leftward movement of the input rod  256  compresses the input rod spring  136  and firmly brings the pin  252  into contact with the sealing member  132 . The sealing member  132  compresses the control valve spring  208 , which moves the inlet member  210  into the fluid channel  207  so that the inlet member  210  approaches the end of a cavity defined by the fluid channel  207 . Meanwhile, the input rod spring  126  compresses and the seat  261  of the collar  254  approaches the second seat  238 . 
     The inlet member  210 , the cavity defined by the fluid channel  207 , and the distance between the collar  254  and the second seat  238 , as depicted in  FIG. 1 , are dimensioned so that when the inlet member  210  bottoms out in the cavity defined by the fluid channel  207 , the seat  261  makes contact with the second seat  238  of the transfer piston actuator  232 . 
     As the seat  261  comes into contact with the second seat  238 , a direct linkage between the brake pedal (not shown) and the transfer piston actuator  110  establishes. This linkage includes the input shaft  140 , the input rod  256 , the collar  254 , the seat  261 , the second seat  238 , and the body portion  201 . 
     With the direct linkage established, further movement of the input rod  256  moves the transfer piston assembly  110  leftward. Since the master cylinder piston assembly  108  has not moved (due to lack of pressure in the boost chamber  128 , and the spring constant of the spring  135  being higher than the input rod spring  136 ), movement of the transfer piston assembly  110  eliminates the gap  134 , as a front portion of the nose  202  comes in contact with the end surface  166 , causing the spring  136  to compress. Further leftward movement of the input rod  256 , results in leftward movement of the master cylinder piston assembly  108  which generates the desired braking function. 
     In an alternative embodiment, the second braking circuit (not shown) may be in fluid communication with a second master cylinder chamber (not shown) as is known in a tandem master cylinder assembly. In this embodiment, the fluid in the second master cylinder chamber (not shown) may be pressurized by leftward movement of a second master cylinder piston (not shown) which is mechanically coupled (via a spring) or fluidly coupled to the master cylinder piston assembly  108 . In this embodiment, the second downstream braking circuit (not shown) may also provide the desired braking function in the failure mode. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.