Abstract:
An improved hydraulic brake system for a vehicle is proposed which includes a hydraulic pressure supply unit for supplying hydraulic pressure from a pressure adjusting valve or an hydraulic pressure source to a pressure chamber of a master cylinder. In this type of brake system, when the hydraulic pressure supply unit is activated, the master piston of the master cylinder could retract to its original position, thus bringing the master pressure chamber into communication with the atmospheric reservoir. If this happens, the master cylinder pressure output will be lost completely. An inexpensive solution to this problem is proposed. A piston retraction restricting member is provided in a pressure chamber for applying pressure to the master piston. The member is adapted to move to a predetermined position under the pressure in the pressure chamber to engage and stop the master piston before the master piston retracts to a position where the master pressure chamber is brought into communication with the atmospheric reservoir through a hole.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to an inexpensive hydraulic brake system for vehicles that permit antilock brake control and vehicle stability control.  
           [0002]    A vehicle hydraulic brake system with which antilock brake control (ABS) and vehicle stability control (VSC) are possible includes a hydraulic pressure source having a power pump for generating a predetermined hydraulic pressure, a master cylinder for generating hydraulic pressure corresponding to a force applied to the brake by a driver and/or the operation of automatic pressurizing means, wheel cylinders actuated by the hydraulic pressure applied from the master cylinder for applying braking force to the vehicle wheels, and wheel cylinder pressure control valves disposed in hydraulic lines that connect the master cylinder to the wheel cylinders, for adjusting pressure in the wheel cylinders.  
           [0003]    Also known is a brake system to which is further added a pressure adjusting valve for adjusting the hydraulic pressure supplied from the hydraulic pressure source to a value corresponding to the brake operation and/or actuation of the automatic pressurizing means.  
           [0004]    These hydraulic brake devices include a controller (that is, electronic control unit) that judges the necessity of wheel cylinder pressure adjustment based on the information from various sensors including wheel speed sensors, and if such adjustment is determined to be necessary, controls the wheel cylinder pressure control valves. For example, if it judges it necessary to reduce the pressure of the wheel cylinders, the controller will activate the wheel cylinder pressure control valves to close the hydraulic pressure supply lines to the wheel cylinders and open the discharge lines from the wheel cylinders.  
           [0005]    The pressures in the wheel cylinders thus fall. Brake fluid discharged from the wheel cylinders is released into the atmospheric reservoir. That is, during every pressure reduction phase of such electronic brake control, i.e. computer-controlled brake operation, brake fluid is discharged into the reservoir, so that the piston of the master cylinder (hereinafter simply “master piston”) gradually advances until it abuts the end wall of the cylinder. Once the master piston abuts the cylinder end wall, it is impossible to supply brake pressure any more from the master cylinder to the wheel cylinder.  
           [0006]    The hydraulic brake system disclosed in Japanese patent publication 59-130769 has a hydraulic pressure supply means for introducing the pressure fluid from the pressure adjusting valve into the hydraulic line connecting the master cylinder with the wheel cylinders, if part of fluid in the above hydraulic line is lost and the amount of the fluid in this line is determined to be insufficient.  
           [0007]    The hydraulic brake system disclosed in this publication includes a solenoid valve for closing the line connecting the hydraulic pressure supply means to the master cylinder, and/or a switch (stroke sensor) for monitoring the stroke of the master piston.  
           [0008]    Once hydraulic pressure is supplied from a pressure adjusting valve into the hydraulic line connecting the master cylinder with the wheel cylinders, the difference between the pressure in a pressure chamber which is applied to the master piston in such a direction as to advance the piston and the pressure in the pressure chamber in the master cylinder (hereinafter “master pressure chamber”) will disappear, so that no pressure acts on the piston to retract it. Since the pressure difference is gone, unless any means for checking the retraction of the master piston is provided, the master cylinder would be pushed back under the force of the return spring to a position where the master pressure chamber communicates with the master cylinder reservoir. If this happens, the pressure output of the master cylinder will disappear.  
           [0009]    Thus, the above publication proposes to close the line leading to the master cylinder with a solenoid valve to prevent the hydraulic pressure supplied through the hydraulic pressure supply means from flowing into the master hydraulic pressure chamber. In another embodiment, this publication proposes to detect the position of the master piston to temporarily stop the supply of brake fluid from the pressure adjusting valve before the master piston returns to a position where the master pressure chamber re-communicates with the reservoir for the master cylinder.  
           [0010]    The hydraulic brake system disclosed in the Japanese patent publication 59-130769 needs expensive elements, such as a solenoid valve for closing the line between the master cylinder and the hydraulic pressure supply means and/or a sensor for monitoring the stroke of the master piston.  
           [0011]    Another conventional brake system is adapted to release brake fluid discharged from the wheel cylinders into a low-pressure reservoir, draw up brake fluid in the low-pressure reservoir by means of a power pump and return the thus sucked up brake fluid into a line between the master cylinder and a master cylinder pressure control valve. This type of brake system requires another power pump for returning brake fluid in addition to a power pump used in the hydraulic pressure source. This pushes up the cost of the entire system.  
           [0012]    An object of this invention is to provide an inexpensive hydraulic brake system for vehicles which permits antilock brake control, vehicle stability control and other electronic brake control.  
         SUMMARY OF THE INVENTION  
         [0013]    According to the invention, there is provided a vehicle hydraulic brake system comprising a hydraulic pressure source for generating a predetermined hydraulic pressure, a pressure adjusting valve for adjusting the hydraulic pressure supplied from the hydraulic pressure source to a value corresponding to a manual brake operation and/or an automatic brake control, a pressure chamber into which the output pressure from the pressure adjusting valve is introduced, a master cylinder including a master piston to which the hydraulic pressure in the pressure chamber is applied, wheel cylinders activated by the output pressure from the master cylinder to apply braking force to wheels of the vehicle, wheel cylinder pressure control valves provided in a hydraulic line connecting the master cylinder to the wheel cylinders for adjusting the hydraulic pressure in the wheel cylinders, and a hydraulic pressure supply unit for supplying the output pressure from the pressure chamber into a hydraulic line connecting the master cylinder to the wheel cylinder pressure control valves, characterized in that there is provided a piston retracting restricting member for preventing the master piston from retracting when the hydraulic pressure supply unit is activated and before the master piston returns to the original position.  
           [0014]    According to the present invention, there is also provided a vehicle hydraulic brake system comprising a hydraulic pressure source for generating a predetermined hydraulic pressure, an atmospheric reservoir, control valves, a pressure chamber connected through the control valves to the hydraulic pressure source and the atmospheric reservoir, a master cylinder including a master piston to which the hydraulic pressure in the pressure chamber is applied, wheel cylinders activated by the output pressure from the master cylinder to apply braking force to wheels of the vehicle, wheel cylinder pressure control valves provided in a hydraulic line connecting the master cylinder to the wheel cylinders for adjusting the hydraulic pressure in the wheel cylinders, and a hydraulic pressure supply unit for supplying the output pressure from the pressure chamber into a hydraulic line connecting the master cylinder to the wheel cylinder pressure control valves, characterized in that there is provided a piston retracting restricting member for preventing the master piston from retracting when the hydraulic pressure supply unit is activated and before the master piston returns to the original position.  
           [0015]    For economical reasons, said member is preferably actuated by the hydraulic pressure in the pressure chamber and is deactivated from its function of preventing the master piston from retracting when the pressure in the pressure chamber has been released.  
           [0016]    Preferably, the member is adapted to prevent the master piston at a predetermined position, particularly at a slightly advanced position than a position where the hydraulic pressure chamber of the master cylinder is brought into communication with the atmospheric reservoir.  
           [0017]    The master cylinder may be a tandem master cylinder comprising the master piston, a floating piston provided in front of the master piston, a first return spring provided between the master piston and the floating piston, a second return spring provided between the floating piston and an end wall of a cylinder of the master cylinder, and an assembly for restricting the expansion of the first return spring to a predetermined length, the first return spring having a larger mounting load than that of the second return spring.  
           [0018]    When brake pressure is repeatedly increased and reduced during antilock brake control, vehicle stability control, etc., pressure in the line (first hydraulic line) connecting the master cylinder to the wheel cylinders gradually decreases. When the controller detects this situation, it activates the hydraulic pressure supply unit to open the line connecting the pressure chamber to the first hydraulic line, thereby supplying hydraulic pressure in the pressure chamber (fluid pressure from the pressure adjusting valve or the hydraulic pressure source) to the first hydraulic line.  
           [0019]    When the pressure of the pressure chamber is supplied into the master pressure chamber, the master piston begins to retract. The piston retraction restricting member engages and stops the piston at a predetermined position. This member may be a simple tubular member. Still, it can reliably stop the retraction of the master piston before the master pressure chamber communicates with the atmospheric reservoir. This simple member eliminates the need for more expensive solenoid valves and stroke sensors as used in the prior art described above. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which:  
         [0021]    [0021]FIG. 1 is a view showing the entire hydraulic brake system according to this invention;  
         [0022]    [0022]FIG. 2 is an enlarged sectional view of the hydraulic pressure adjusting device of the brake system of FIG. 1; and  
         [0023]    [0023]FIG. 3 is a view of the hydraulic brake system of another embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    Now referring to FIGS.  1 - 3 , the embodiments of this invention will be described.  
         [0025]    The hydraulic brake system  1  shown in FIG. 1 includes a hydraulic pressure source  2  having a power pump  2   a , a pressure accumulator  2   b  and a pressure sensor  2   c ; a hydraulic pressure adjusting unit  3  including a master cylinder  4  and a pressure adjusting valve  5 ; an atmospheric reservoir  6  for supplying brake fluid to the hydraulic pressure source  2  and the master cylinder  4 , and wheel cylinders W 1 -W 4  for applying braking force to the respective vehicle wheels. The brake system  1  further includes wheel cylinder pressure control valves  8   -1  and  8   -2  and a pressure sensor  9  disposed in a first hydraulic line  7  that connects the master cylinder  4  to the wheel cylinders W 1  and W 2 , and wheel cylinder pressure control valves  8   -3  and  8   -4 , a solenoid valve  11  and a pressure sensor  12  disposed in a second hydraulic line  10  connecting the pressure adjusting valve  5  to the wheel cylinders W 3  and W 4 . The brake system  1  further includes two proportional solenoid valves  13  and  14  (which produce a differential pressure corresponding to an electronic command), a solenoid valve  15  disposed in a hydraulic line extending from a pressure chamber C 2  to the first hydraulic line  7 , a piston retraction restricting member  16  provided in the pressure chamber C 2 , a controller (electronic control unit)  17  for controlling the entire brake system  1 , and various sensors (only pressure sensors shown) for detecting the behavior of the vehicle and the status of the drive train and sending detection signals to the controller  17 . The proportional solenoid valve  13  is disposed in a hydraulic line connecting the delivery port of the pump  2   a  to the hydraulic line  10  not through the solenoid valve  11 . The proportional solenoid valve  14  is disposed in a pressure-reducing hydraulic line connecting the atmospheric reservoir  16  to the hydraulic line  10  not through the solenoid valve  11 .  
         [0026]    [0026]FIG. 2 is an enlarged view of the hydraulic pressure adjusting unit  3 . It includes a cylinder  18 , an auxiliary piston  19  mounted in the cylinder  18 , a stroke simulator  21  comprising a simulator piston  21   a  operatively associated with a brake operating member  20  (such as a brake pedal shown) and a biasing member  21   b  that imparts to the simulator piston  21   a  a stroke corresponding to the brake operation, and a distributor  22  for distributing the brake operating force applied thereto to the pressure adjusting valve  5  and the auxiliary piston  19  through the stroke simulator  21 .  
         [0027]    The master cylinder  4  comprises a master piston  4   a  having its front surface disposed in a master hydraulic pressure chamber C 1  and its rear surface disposed in the pressure chamber C 2 , a return spring  4   b  for the master piston  4   a , and a seal  4   c  for sealing the outer periphery of the piston  4   a.    
         [0028]    The pressure adjusting valve  5  includes a spool  5   a  to change over the increase, decrease and keeping of the output pressure. The spool  5   a  is adapted to move to a position where the sum of a thrust force corresponding to the hydraulic pressure in a pressure chamber C 4  and the force of the return spring  5   b  balances with the force transmitted from the brake operating member  20  to the spool  5   a  through the distributor  22 . The auxiliary piston  19  is formed with an input port P 01 , an output port P 02  and a pressure reduction port P 03 . According to the position of the spool  5   a , the output pressure at the output port P 02  can be increased, reduced or maintained. That is, according to the position of the spool  5   a , the output port P 02  is selectively connected to the input port P 01  or pressure reduction port P 03  or to neither of them. While the input port P 01  is in communication with the output port P 02  through a passage in the spool  5   a , the degree of opening of a valve portion defined between a shoulder of the spool  5   a  and the input port P 01  is adjusted by slight movement of the spool  5   a . Similarly, while the output port P 02  is in communication with the pressure reduction port P 03  through the passage in the spool  5   a , the degree of opening of a valve portion defined between a shoulder of the spool  5   a  and the pressure reduction port P 03  is adjusted by slight movement of the spool  5   a . Thus, the hydraulic pressure P 1  supplied from the hydraulic pressure source  2  is adjusted to a hydraulic pressure P 2  corresponding to the force applied to the brake operating member  20  and the hydraulic pressure P 2  is supplied to the wheel cylinders W 3  and W 4  through the fluid chambers C 4  and C 3 . Since the pressure adjusting valve  5  is known in the art, its detailed description is omitted.  
         [0029]    The force distributor  22  comprises a cup member  22   a , a rubber disk  22   b  provided in the cup member  22   a , a force transmission member  22   c , a tubular member  22   e  having one end thereof supported by the auxiliary piston  19  and the other end carrying a resin ring  22   d  and inserted in the cup member  22   a  so as to oppose the rubber disk  22   b  with a gap g formed therebetween, and a steel ball  22   f  mounted to the force transmission member  22   c  so as to abut the spool  5   a.    
         [0030]    In the initial stage of brake operation, the force applied from the brake operating member  20  is transmitted only to the pressure adjusting valve  5  through the rubber disk  22   b , transmission member  22   c  and steel ball  22   f  of the force distributor  22 . When the brake operating force exceeds a threshold, the rubber disk  22   b  will be elastically deformed to fill the gap g, thus coming into contact with the resin ring  22   d . Once the rubber disk  22   b  contacts the resin ring  22   d , part of the brake operating force is transmitted to the auxiliary piston  19  through the tubular member  22   e.    
         [0031]    Since the brake operating force is transmitted only to the adjusting valve  5  in the initial stage of brake operation, it is possible to quickly increase the braking force, that is, to give jumping characteristics to the brake system. The inner diameter of the tubular member  22   e  and the outer diameter of the force transmission member  22   c  determine the ratio between the force transmitted to the pressure adjusting valve  5  and the force transmitted to the auxiliary piston  19 . The lengths of these members determine the timing at which the distribution of the brake operating force starts. Thus, one or both of these parameters can be changed by replacing the tubular member  22   e  and the force transmission member  22   c  with ones having different diameters and/or different lengths.  
         [0032]    In this regard, the force distributor  22  is a preferable element. But it may be omitted. If omitted, the brake operating force is directly transmitted to the pressure adjusting valve  5 .  
         [0033]    The auxiliary piston  19  is provided to directly transmit the brake operating force to the master piston  4   a  in case the hydraulic pressure source  2  or a line connecting thereto fails. The hydraulic pressure output from the pressure adjusting valve  5  is introduced into the pressure chamber C 2  to push the auxiliary piston  19  rightwardly in the figure and keep it in the illustrated position. However, if the hydraulic pressure source  2  fails and no pressure is produced in the pressure chamber C 2 , the auxiliary piston  19  will be pushed leftwardly by the force transmitted from the brake operating member  20  through the force distributor  19 , thus applying pressure to the master piston  4   a . The hydraulic pressure thus produced in the master cylinder  4  is used to produce the braking force. Thus, even if the hydraulic pressure source  2  fails, it is still possible to apply brake.  
         [0034]    The solenoid valve  11  and the proportional solenoid valves  13 ,  14  in FIG. 1 are provided to allow regenerative cooperative braking control and automatic brake control (such as vehicle stability control or car-to-car distance control), which does not depend on brake operation of a driver.  
         [0035]    In regenerative cooperative brake control used in an electric vehicle, priority is given to regenerative braking. This means that while regenerative braking is on, it is necessary to reduce the braking force originating from hydraulic pressure by an amount corresponding to the regenerative braking force set for the wheels.  
         [0036]    The controller  17  calculates the optimum regenerative braking force to be generated based on information from elements involved in regenerative braking, such as sensors, and controls the solenoid valve  11  and the proportional solenoid valves  13  and  14  so that the difference between the hydraulic pressure P 2  in the fluid chamber C 3 , which is detected by the pressure sensor  12 , and the hydraulic pressure P 3  in the master hydraulic pressure chamber C 1 , which is detected by the pressure sensor  9 , will be equal to the pressure corresponding to the calculated regenerative braking force.  
         [0037]    With this arrangement, reduced hydraulic pressure is supplied to the wheel cylinders W 3  and W 4 . Also, since due to this pressure reduction, the hydraulic pressure in the pressure chamber C 2  also drops, the hydraulic pressure output of the master cylinder  4  will also drop, so that the braking force applied to the wheels by the wheel cylinders W 1 -W 4  also drops by an amount corresponding to the regenerative braking force.  
         [0038]    Even during such regenerative cooperative brake control, the auxiliary piston  19  is biased rightwardly in FIG. 2 by the hydraulic pressure in the fluid chamber C 3 . Thus, provided the hydraulic pressure source  2  is functioning normally, the piston  19  remains stationary at the position shown in FIG. 1 even during regenerative cooperative brake control.  
         [0039]    The proportional solenoid valve  13  permits automatic brake control, i.e. brake control with the brake not operated by the driver. In such automatic brake control, the controller  17  closes the solenoid valve  11  and opens the proportional solenoid valve  13  to apply hydraulic pressure output of the hydraulic pressure source  2  to the wheel cylinders W 3  and W 4 . The hydraulic pressure also flows into the pressure chamber C 2 , so that the master cylinder  4  is also pressurized. This activates the wheel cylinders W 1  and W 2  which give braking force to the wheels.  
         [0040]    Each of the wheel cylinder pressure control valves  8   -1  to  8   -4  shown comprises a solenoid valve Va having a check valve and adapted to open and close the line leading to the wheel cylinder, and a solenoid valve Vb for opening and closing the discharge line from the wheel cylinder. But instead of the valves Va and Vb, a single solenoid changeover valve having both functions may be used.  
         [0041]    Brake fluid discharged from each wheel cylinder through the solenoid valve Vb flows through a discharge line  23  back to the atmospheric reservoir  6 .  
         [0042]    Thus, during antilock brake control, vehicle stability control, and other brake control which involve repeated pressure increase and reduction, the volume of the master hydraulic pressure chamber C 1  tends to decrease gradually. This means that the master piston  4   a  gradually advances and will eventually abut the end wall of the cylinder  18 , unless any preventive measures are taken. Once the master piston  4   a  abuts the end wall of the cylinder  18 , no hydraulic pressure can be supplied from the master cylinder  4  any more.  
         [0043]    In order to prevent the master piston  4   a  from abutting the end wall of the cylinder  18 , the hydraulic brake system  1  of FIG. 1 has a hydraulic pressure supply means (solenoid valve  15 ) for supplying, when necessary, hydraulic pressure of the pressure chamber C 2  (hydraulic pressure output of the pressure adjusting valve  5  or hydraulic pressure source  2 ) to the hydraulic pressure line  7 .  
         [0044]    When the solenoid valve  15  is opened, the fluid pressure in the pressure chamber C 2  is supplied to the fluid line  7  and the pressure chamber C 2  and the master hydraulic pressure chamber C 1  communicate with each other through the line  7 . Thus, the difference between the pressure in the master hydraulic chamber C 1  and the pressure in the pressure chamber C 2 , which act on both ends of the master piston  4   a  to bias the master piston in opposite directions, disappears. The master piston  4   a  is thus pushed back by the force of the return spring  4   b.    
         [0045]    If the master piston  4   a  were allowed to retract to the original position shown in FIG. 1, the master hydraulic pressure chamber C 1  would be brought into communication with the atmospheric reservoir  6  through a hole h formed in the cylinder  18 , resulting in the loss of hydraulic pressure in the master hydraulic pressure chamber C 1 . Of course, this must not happen because brake control is now being carried out. The present invention proposes an inexpensive solution to this problem, which comprises a piston retraction restricting member  16 .  
         [0046]    The piston retraction restricting member  16  is a tubular piston mounted between the outer periphery of the master piston  4   a  and the inner surface of the pressure chamber C 2 . It is movable under the pressure in the pressure chamber C 2  until it abuts the end wall  24  of the atmospheric chamber.  
         [0047]    While the solenoid valve  15  is open and the pressure difference between the master hydraulic pressure chamber C 1  and the pressure chamber C 2  is zero, the piston retraction restricting member  16  is kept in abutment with the end wall  24  to restrict the retraction of the master piston  4   a . The member  16  has a stopper  16   a  adapted to engage the shoulder of the master piston  4   a  to perform its function. The master piston  4   a  and the member  16  are arranged such that when, with the member  16  in abutment with the end wall  24 , the master piston  4   a  has moved rightwardly in FIG. 1 by the return spring  4   b  to a position displaced leftwardly from its original position of FIG. 1 by a distance L, the shoulder of the master piston  4   a  is engaged by the stopper  16   a  of the member  16 . In this state, the master hydraulic pressure chamber C 1  will never open to the atmospheric reservoir  6  through the hole h.  
         [0048]    When the brake pedal is released or when computer-initiated brake control ends, so that the pressure in the pressure chamber C 2  disappears, the master piston  4   a  will be pushed back to the original position shown in FIG. 1 together with the member  16  under the force of the return spring  4   b.    
         [0049]    In order to minimize the stroke L of the member  16  and thus the entire length of the brake system, the master piston  4   a  and the retraction restricting member  16  are preferably arranged such that the master piston  4   a  will be stopped by the member  16  immediately before a point where the master hydraulic pressure chamber C 1  is brought into communication with the atmospheric reservoir  6 .  
         [0050]    [0050]FIG. 3 shows a hydraulic brake system of the second embodiment. This brake system includes a hydraulic pressure adjusting device  3 A having a tandem master cylinder.  
         [0051]    The tandem master cylinder  4 A includes a master piston  4   a   -1  having one end thereof disposed in a first master hydraulic pressure chamber C 1   -1  and the other end disposed in the pressure chamber C 2 , a floating piston  4   a   -2  (which is another master piston) having one end thereof disposed in a second master hydraulic pressure chamber C 1   -2  and the other end disposed in the first master hydraulic pressure chamber C 1   -1  and provided in front of the master piston  4   a , a first return spring  4   b   -1  provided between the master piston  4   a   -1  and the floating piston  4   a   -2 , and a second return spring  4   b   -2  provided between the floating piston  4   a   -2  and the end wall of the cylinder  18 .  
         [0052]    A support pin  4   d  is fixed to the master piston  4   a   -1  so as to extend toward the floating piston  4   a   -2 . A retainer  4   e  is slidably fitted on the support pin  4   d , which has a large-diameter free end which serves to engage the free end of the retainer  4   e , thereby keeping the retainer  4   e  from coming off the pin  4   d . The retainer  4   e  has its other end in abutment with the floating piston  4   a   -2 . One end of the first return spring  4   b   -1  is mounted on the retainer  4   e . Thus, the spring  4   b   -1  cannot expand beyond the point at which the large-diameter end of the pin  4   d  is in engagement with the free end of the retainer  4   e.    
         [0053]    Also, the mounting load for the first return spring  4   b   -1  is set to be larger than the mounting load for the second return spring  4   b   -2 .  
         [0054]    In the pressure chamber C 2 , a piston retraction restricting member  16  which is the same as the one shown in FIG. 1 is provided. In the hydraulic brake system of FIG. 3 the hydraulic pressure produced in the first master pressure chamber C 1   -1  of the tandem master cylinder  4 A is applied to the wheel cylinders W 3  and W 4  through the second hydraulic line  10  and the hydraulic pressure produced in the second master pressure chamber C 1   -2  is applied to the wheel cylinders W 1  and W 2  through the first hydraulic line  7 . Thus, during antilock brake control, vehicle stability control, and other brake control which involve repeated pressure increase and pressure reduction, the master piston  4   a   -1  and the floating piston  4   a   -2  will gradually advance, so that the master pressure chambers C 1   -1  and C 1   -2  may be brought into communication with the atmospheric reservoir  6  through holes h 1  formed in the master piston  4   a   -1  and the floating piston  4   a   -2  and holes h formed in the cylinder  18 . If this should happen, it becomes impossible to supply fluid pressure to either of the first and second hydraulic lines. To prevent such a failure, solenoid valves  15  for supplying hydraulic pressure are provided in the first and second hydraulic lines.  
         [0055]    Otherwise, the second embodiment is structurally the same as the first embodiment. Thus, like elements are denoted by like numerals and description is omitted.  
         [0056]    When the pressure output from the pressure chamber C 2  is supplied through the solenoid valves  15  into the hydraulic lines  7  and  10 , the piston retraction restriction member  16  engages the master piston  4   a   -1 , thereby preventing it from moving rightwardly any further. In this state, the mounting load of the first return spring  4   b   -1 , which biases the floating piston  4   a   -2  leftwardly in FIG. 3, is set to be greater than that of the second return spring  4   b   -1 , which biases the floating spring  4   b   -1  rightwardly in FIG. 3. Thus, the first return spring  4   b   -1  is not compressed by the second return spring  4   b   -2 . This means that the floating piston  4   a   -2 , too, remains stationary at this position. Thus, it is possible to maintain the hydraulic pressures both in the first and second master pressure chambers C 1   -1  and C 1   -2 .  
         [0057]    In the description of the embodiments, pressure fluid is supplied from the pressure chamber C 3  into the master pressure chamber or chambers. But instead, pressure fluid may be directly supplied from the pressure source  2  to the hydraulic pressure line at the master cylinder side by opening the valves  13  and  15  while closing the valve  11 . In the latter case, the pressure adjusting valve  5  is not necessary.  
         [0058]    The piston retraction restricting member as described above eliminates the necessity of providing more expensive conventional elements, such as a solenoid valve for closing the line between the hydraulic pressure supply means and the master cylinder, and a stroke sensor for detecting the position of the master piston. This reduces the entire cost of the brake system.