Master cylinder apparatus

A master cylinder apparatus for a BBW system is provided, which eliminates a risk of brake delay, even with the use of a plunger-type master cylinder, and which therefore is able to achieve a reduction in size of a master cylinder. The master cylinder apparatus comprises a tandem-type master cylinder connected to wheel cylinders through fail-safe valves and a stroke simulator for ensuring a desired stroke of a brake pedal upon receiving a brake fluid introduced from a fluid pressure chamber in the master cylinder. The master cylinder is arranged in the form of a plunger-type master cylinder in which cup seals are provided on an inner surface of a bore of the cylinder body. A spring force of a second return spring for a secondary piston is set to be larger than that of a first return spring for a primary piston. A retracted position of the secondary piston is limited by a stopper pin extending through an oblong hole formed in the secondary piston.

BACKGROUND OF THE INVENTION

The present invention relates to a brake system for an automobile. More specifically, the present invention relates to a master cylinder apparatus used in a brake fluid pressure controlling system for electrically controlling a fluid pressure supplied to wheel cylinders; i.e., a so-called brake-by-wire (BBW) system.

A master cylinder apparatus for a BBW system comprises a master cylinder adapted to be connected to wheel cylinders through fail-safe valves and a stroke simulator for ensuring a desired stroke of a brake pedal upon receiving a brake fluid introduced from the master cylinder. In the event of failure of the BBW system, the fail-safe valves are opened, and the fluid pressure generated in the master cylinder is supplied to the wheel cylinders.

This type of related art is disclosed in, for example, U.S. Pat. No. 6,192,685 B1.

However, the master cylinder apparatus of U.S. Pat. No. 6,192,685 B1 is configured such that supply of brake fluid from a reservoir to a primary-side pressure chamber and a secondary-side pressure chamber is controlled by means of a center valve. Consequently, axial dimensions of the primary piston and the secondary piston are large, and an overall size of a master cylinder is also large.

To counter this problem, it has been suggested to use a plunger-type master cylinder in which cup seals are provided on an inner surface of a bore of a cylinder body, so as to seal outer circumferential surfaces of a primary piston and a secondary piston, each of which is subject to a sliding movement in the cylinder body. In the case of a master cylinder apparatus for a BBW system, it is generally required to set a spring force of a return spring for biasing a secondary piston to be larger than that of a return spring for biasing a primary piston, so as to limit a force that a driver is required to apply to a brake pedal. In such a configuration, since a spring force for biasing the secondary piston must be made large, and a return position (a retracted position) of the secondary piston during non-braking is therefore variable, a possibility exists that in the event of a failure of the BBW system, an invalid stroke for operating the secondary piston will be large, resulting in a delay in braking.

SUMMARY OF THE INVENTION

The present invention has been made with a view to overcoming the drawbacks of the prior art, as stated above. It is therefore an object of the present invention to provide a master cylinder apparatus which eliminates a risk of brake delay in the event of failure of a BBW system, even with the use of a plunger-type master cylinder, and which therefore is able to safely achieve a reduction in size of a master cylinder.

In order to achieve the above-mentioned object, the present invention provides a master cylinder apparatus comprising:

a tandem-type master cylinder adapted to be connected to wheel cylinders through fail-safe valves; and

a stroke simulator for ensuring a desired stroke of a brake pedal upon receiving a brake fluid introduced from a primary-side fluid pressure chamber formed in the master cylinder,

the tandem-type master cylinder comprises:

a cylinder body;

a primary piston and a secondary piston arranged in an axial direction of the cylinder body and being capable of sliding movement in the cylinder body;

the primary-side fluid pressure chamber, which is defined between the primary piston and the secondary piston;

a secondary-side fluid pressure chamber defined between the secondary piston and the cylinder body;

a first circumferential groove and a second circumferential groove formed in an inner surface of a bore of the cylinder body, the first circumferential groove and the second circumferential groove being spaced apart from each other in the axial direction of the cylinder body;

a first cup seal provided in the first circumferential groove and adapted to seal an outer circumferential surface of the primary piston;

a second cup seal provided in the second circumferential groove and adapted to seal an outer circumferential surface of the secondary piston;

a first return spring provided in the primary-side fluid pressure chamber so as to bias the primary piston in a direction for retraction;

a second return spring provided in the secondary-side fluid pressure chamber so as to bias the secondary piston in the direction for retraction; and

a limiting means provided in the cylinder body so as to limit a retracted position of the secondary piston.

In this master cylinder apparatus, the second return spring for biasing the secondary piston may have a larger spring force than the first return spring for biasing the primary piston. The limiting means may comprise a stopper pin which extends across the bore of the cylinder body, the stopper pin being provided in an oblong hole extending through the secondary piston.

In the master cylinder apparatus arranged as mentioned above, there is provided a limiting means adapted to limit a retracted position of a secondary piston; as a result of which a retracted position of the secondary piston remains constant, even in a case that a spring force of a return spring for biasing the secondary piston is larger than that of a return spring for biasing a primary piston. By this means, an invalid stroke of the secondary piston can be markedly reduced. Further, by use of a plunger-type master cylinder, an axial dimension of a master cylinder can also be reduced.

Since the retracted position of the secondary piston is limited by the limiting means, an invalid stroke of the secondary piston is reduced. Therefore, in the event of failure of the BBW system, brake delay does not occur, and the apparatus is made highly reliable. Further, since an axial dimension of a master cylinder is reduced by use of a plunger-type master cylinder, the master cylinder apparatus is easily mountable on a vehicle.

DETAILED DESCRIPTION OF THE INVENTION

Hereafter, an embodiment of the present invention is described, with reference to the accompanying drawings.

FIGS. 1 and 2show the entire structure of a master cylinder apparatus according to an embodiment of the present invention. A master cylinder apparatus1is used in a BBW system described above. The master cylinder apparatus1is connected to wheel cylinders (not shown) through fail-safe valves2A and2B. The master cylinder apparatus1comprises a tandem-type master cylinder4for generating a fluid pressure corresponding to a force applied to a brake pedal3, and a stroke simulator5for ensuring a desired stroke of the brake pedal3. The stroke simulator5is externally mounted on a cylinder body10of the master cylinder4. A first fluid pressure chamber (a primary-side fluid pressure chamber)13is defined between a primary piston11and a secondary piston12provided in the master cylinder4. When the brake pedal3is operated, a brake fluid in the first fluid pressure chamber13is introduced into the stroke simulator5, to thereby ensure a desired stroke of the brake pedal3.

The master cylinder apparatus1further comprises an opening/closing means7and a stroke sensor8. The opening/closing means7is provided in a simulator passage6which allows communication between the first fluid pressure chamber13in the master cylinder4and the stroke simulator5. The stroke sensor8is adapted to detect a stroke of the primary piston11(a piston stroke), which moves in the master cylinder4in accordance with the movement of the brake pedal3. The BBW system comprises a fluid pressure controlling means including a fluid pressure source, a fluid pressure control valve and an electronic control unit, etc., in addition to the master cylinder apparatus1. Normally, the fluid pressure controlling means controls a fluid pressure supplied to the wheel cylinders, on the basis of a detection signal emitted from the stroke sensor8.

The master cylinder4is arranged in the form of a plunger-type master cylinder. The cylinder body10is arranged in the form of a cylinder having one end closed, as shown inFIG. 3. The primary piston11and the secondary piston12are slidably provided in a bore14of the cylinder body10.

A forward end (an end for insertion into the bore14) of the primary piston11forms a cup-like portion11a. The first fluid pressure chamber13is defined between the cup-like portion11aof the primary piston11and the secondary piston12. A forward end of the secondary piston12also forms a cup-like portion12a. A second fluid pressure chamber (a secondary-side fluid pressure chamber)15is defined between the cup-like portion12aand the closed end of the cylinder body10. The cylinder body10includes a first discharge port16for supplying brake fluid from the first fluid pressure chamber13to the corresponding wheel cylinders, and a second discharge port17for supplying brake fluid from the second fluid pressure chamber15to the corresponding wheel cylinders. The first discharge port16opens into a longitudinal groove18formed in an inner surface of the bore14of the cylinder body10; and the second discharge port17opens into a longitudinal groove19formed in the inner surface of the bore14of the cylinder body10.

A first return spring20is provided between a bottom of the cup-like portion11aof the primary piston11and the secondary piston12. A second return spring21is provided between a bottom of the cup-like portion12aof the secondary piston12and the closed end of the cylinder body10. Normally, spring forces of the first and second return springs20and21bias each of the pistons11and12in a direction away from the bore14.

The spring force of the second return spring21for biasing the secondary piston12is set to be larger than that of the first return spring20for biasing the primary piston11.

A rear end portion of the cylinder body10is connected to a piston guide23in the form of a cylinder having one end closed, by means of a retaining member22threadably engaged with the rear end portion of the cylinder body10. A bottom plate of the piston guide23prevents the primary piston11from separating from the bore14, while limiting a retracted position of the primary piston11. A retracted position of the secondary piston12is limited by a stopper pin (a limiting means)25which is inserted into a diametrical hole (an oblong hole)24extending through a solid portion of the secondary piston12. As shown inFIG. 4, the stopper pin25extends across the bore14, with a base end portion thereof being threadably engaged with a wall of the cylinder body10.

A rear end (opposite to the end for insertion) of the primary piston11includes a recess11bextending along the axis of the primary piston11. An input shaft26extending from the brake pedal3is inserted into the recess11b. The input shaft26is locked in the recess11bin a state such that a spherical portion26aformed at a forward end of the input shaft26abuts against a bottom part of the recess11b. The primary piston11is adapted to advance toward a closed end of the bore14under a force applied from the brake pedal3through the input shaft26.

The inner surface of the bore14of the cylinder body10includes two annular grooves27and28. The annular groove27faces the primary piston11, and the annular groove28faces the secondary piston12. A reservoir port30communicating with a reservoir29mounted on the top of the cylinder body10opens into the annular groove27. A reservoir port31communicating with the reservoir29opens into the annular groove28. The cup-like portion11aof the primary piston11includes a supply port32, and the cup-like portion12aof the secondary piston12includes a supply port33. When the primary piston11and the secondary piston12are located at their respective retracted positions, the supply port32and the supply port33are open to the annular groove27and the annular groove28, respectively. In this state, brake fluid is supplied from the reservoir29to the first fluid pressure chamber13and the second fluid pressure chamber15.

A pair of cup seals34and35are provided on the inner surface of the bore14of the cylinder body10, with the primary-side annular groove27being disposed therebetween. Further, on the inner surface of the bore14of the cylinder body10, a pair of cup seals36and37are provided, with the secondary-side annular groove28being disposed therebetween.

Of the cup seals on the primary side, the cup seal34located on a side of an open end of the bore14serves to seal the first fluid pressure chamber13from the outside. Of the cup seals on the secondary side, the cup seal36located on a side of the open end of the bore14serves to prevent communication between the first fluid pressure chamber13and the second fluid pressure chamber15.

On the other hand, of the cup seals on the primary side, the cup seal35located on a side of the closed end of the bore14serves to prevent flow of the fluid from the first fluid pressure chamber13to the annular groove27communicated with the reservoir29. Of the cup seals on the secondary side, the cup seal37located on a side of the closed end of the bore14serves to prevent flow of the fluid from the second fluid pressure chamber15to the annular groove28communicated with the reservoir29.

The primary-side cup seals34and35and the secondary-side cup seals36and37are respectively provided in annular grooves formed in the inner surface of the bore14of the cylinder body10. The cup seal35on a side of the closed end of the bore14is provided in an annular groove38, which is communicated with the longitudinal groove18formed in the inner surface of the bore14. The cup seal37on a side of the closed end of the bore14is provided in an annular groove39, which is communicated with the longitudinal groove19formed in the inner surface of the bore14. As shown inFIG. 5, the longitudinal groove18, which is communicated with the annular groove38in which the primary-side cup seal35is provided, is shallower than the annular groove38. That is, an outer circumferential edge of the cup seal35abuts against a front wall surface38aof the annular groove38to limit a flow of the fluid from the annular groove27behind the cup seal35to the first fluid pressure chamber13. In contrast, as shown inFIG. 6, the longitudinal groove19, which is communicated with the annular groove39in which the secondary-side cup seal37is provided, has a depth equal to or slightly larger than the depth of the annular groove39, thus allowing flow of the fluid from the annular groove28behind the cup seal37to the second fluid pressure chamber15.

As shown inFIG. 1, the stroke simulator5comprises a simulator body40having a stepped configuration including a small-diameter portion40a, an intermediate-diameter portion40band a large-diameter portion40c. External threads are formed in an outer circumferential surface of the intermediate-diameter portion40b. On the other hand, a boss portion42having a stepped inner surface defining a fitting opening41is projected from the cylinder body10of the master cylinder4. A large-diameter portion of the fitting opening41has internal threads. The simulator body40of the stroke simulator5is directly connected to the cylinder body10by threadably engaging the intermediate-diameter portion40bwith the fitting opening41of the cylinder body10. Thus, the simulator body40of the stroke simulator5is mounted on an exterior surface of the cylinder body10. To connect the simulator body40to the cylinder body10, the small-diameter portion40aat a forward end of the simulator body40is press-fitted into a small-diameter portion of the fitting opening41through a seal member43(seeFIG. 7).

The simulator passage6which communicates the first fluid pressure chamber13of the master cylinder4with the stroke simulator5comprises a port50(described later) formed at a bottom end of the fitting opening41of the cylinder body10, a fluid passage51in the opening/closing means7and a fluid passage52formed in the simulator body40(FIGS. 3 and 7).

As indicated inFIG. 1, the simulator body40of the stroke simulator5includes a bore44having an end wall. A piston46is slidably provided in the bore44through a cup seal45. A pressure chamber S is defined between a forward end (an end for insertion into the bore44) of the piston46and the end wall of the bore44, which pressure chamber is sealed by the cup seal45. The fluid passage52forming the simulator passage6opens into the pressure chamber S. The large-diameter portion40cof the simulator body40has a hollow portion. The bore44is extended so as to form a cylindrical extension44ain the hollow portion of the large-diameter portion40c. The large-diameter portion40cof the simulator body40has an open end on a side opposite to the small-diameter portion40a. The open end of the large-diameter portion40cis closed by a cover plate40′. A spring bearing48is provided at a distal end of the cylindrical extension44aof the large-diameter portion40c, so as to face the cover plate40′. A first spring47is disposed between the spring bearing48and the cover plate40′. One end of the first spring47is seated on the cover plate40′. The other end of the first spring47is received by the spring bearing48. Further, a second spring49having a smaller spring force than the first spring47is provided inside the cylindrical extension44a. The second spring49is interposed between the spring bearing48and a cup-like surface of the piston46and normally biases the piston46in an upward direction. In the stroke simulator5, when the fluid pressure in the pressure chamber S rises, the piston46first moves downward against the spring force of the second spring49and abuts against the spring bearing48. Thereafter, the piston46moves downward against the spring force of the first spring47.

As is clearly shown inFIGS. 7 to 10, the opening/closing means7comprises a poppet valve54for opening and closing the fluid passage52(the simulator passage6) in the simulator body40and a rocking lever55which is movable together with the secondary piston12in the master cylinder4to open and close the poppet valve54. The poppet valve54and the rocking lever55are assembled in a casing56, to form a unit. To assemble the simulator body40to the fitting opening41of the boss portion42of the cylinder body10, the unit is connected between the cylinder body10and the simulator body40through a washer57. As indicated inFIG. 8, the port50forming the simulator passage6on a side of the cylinder body10has an oblong (non-circular) form, which extends in an axial direction of the bore14of the cylinder body10. The casing56includes a C-shaped projection56aprojected upwardly therefrom. The C-shaped projection56ais fitted into the oblong port50while being located close to the front side (on a side of the closed end of the bore14) of the oblong port50. Thus, by means of the oblong port50, the casing56is prevented from rotating.

The poppet valve54comprises a valve seat61, a valve body62adapted to move to and away from the valve seat61and a valve spring63which normally biases the valve body62in a direction for closing the valve. The valve seat61is formed at the bottom of a recess60formed in the simulator body40, so as to surround an opening of the fluid passage52. The fluid passage52is closed when the valve body62is seated on the valve seat61, and is open when the valve body62is separated from the valve seat61. The valve spring63has one end engaged with the casing56, and normally biases the valve body62in the direction for closing the valve. The valve body62is slidably fitted into a through-hole56bformed in the casing56, with a lower end thereof being connected to an elastic member64capable of intimate contact with the valve seat61. An upper end portion of the valve body62includes a constricted portion62aengageable with the rocking lever55. The fluid passage51in the opening/closing means7, which forms the simulator passage6, is an inclined passage formed in the casing56, with one end thereof opening into the through-hole56b. Therefore, when the poppet valve54is open, as shown inFIG. 7, brake fluid in the first fluid pressure chamber13of the master cylinder4is supplied to the stroke simulator5through the port50in the cylinder body10, the fluid passage51in the casing56, the recess60around the valve body62and the fluid passage52in the simulator body40. The material of the elastic member64is not particularly limited, as long as it is capable of intimate contact with the valve seat61. Various materials, such as rubber, resin, etc., may be used for the elastic member64.

As is clearly shown inFIG. 9, the rocking lever55comprises a shaft portion65supported by a shaft bearing portion formed in the casing56, a claw portion66extending radially outwardly from the shaft portion65and a columnar portion67extending from the shaft portion65substantially at a right angle relative to the claw portion66. The claw portion66is engageable with the constricted portion62aof the valve body62. When the rocking lever55is supported by the casing56, an upper end of the columnar portion67of the rocking lever55extends through the port50of the cylinder body10into the bore14. A rear end portion of the secondary piston12includes an annular groove68. The upper end of the columnar portion67is positioned within the annular groove68.

When the secondary piston12is located at the retracted position, a front wall surface69of the annular groove68is positioned close to the rear side of the oblong port50. In this state, the columnar portion67of the rocking lever55is rotated by the front wall surface69in a clockwise direction inFIG. 7, and finally abuts against the front wall surface69and maintains an upright position (FIG. 7). Consequently, the valve body62of the poppet valve54is lifted by the claw portion66and, as shown inFIG. 7, the fluid passage52(the simulator passage6) on a side of the stroke simulator5is opened. When the secondary piston12advances from the retracted position, the front wall surface69moves in a leftward direction inFIG. 7, to thereby release the force of the rocking lever55lifting the valve body62of the poppet valve54. Since the valve body62is biased in the valve-closing direction by means of the valve spring63, the rocking lever55rocks about the shaft portion65, and the fluid passage52is closed, as shown inFIG. 10.

As shown inFIGS. 1 and 3, the stroke sensor8is provided in a cover70connected to a flange portion10aat the rear end portion of the cylinder body10. The stroke sensor8comprises a sensor body71containing a rotation angle detector (not shown), a rotary shaft72extending from the rotation angle detector downward beyond the lower surface of the sensor body71, a sensor arm73having one end fixedly connected to the rotary shaft72, and a sensor pin75. The sensor pin75extends upward from the rear end of the primary piston11and extends through a slit74formed in the piston guide23toward the sensor body71.

As is clearly shown inFIG. 11, an oblong hole76is formed in the other end of the sensor arm73. An upper end portion of the sensor pin75is inserted into the oblong hole76. The sensor pin75is adapted to linearly move, together with the primary piston11, in the slit74formed in the piston guide23. The oblong hole76of the sensor arm73has a length sufficient for ensuring a linear movement of the sensor pin75, and has a width sufficient for ensuring a smooth motion of the sensor pin75. The sensor arm73is biased in a counterclockwise direction as viewed inFIG. 11, by means of a biasing means (not shown). By this arrangement, a wall surface76aon one side of the oblong hole76is always pressed against the sensor pin75. That is, the sensor pin75is adapted to perform linear movement without play in the oblong hole76of the sensor arm73. Therefore, an amount of linear movement of the primary piston11can be accurately converted to an amount of rotation of the rotary shaft72. In this case, as indicated by a solid line inFIG. 14, a substantially linear relationship exists between the amount of linear movement of the primary piston11and the angle of rotation of the rotary shaft72, as a result of which accurate sensing can be stably conducted over an entire length of a stroke.

Instead of the sensor arm73having a straight form shown inFIG. 11, a V-shaped sensor arm73′ as shown inFIG. 12or a curved sensor arm73″ as shown inFIG. 13may be employed.

The sensor arm73′ is bent in a direction outward relative to a line connecting the rotary shaft72and the sensor pin75, and the sensor arm73″ is curved outward relative to the line connecting the rotary shaft72and the sensor pin75. In the sensor arm73″, the oblong hole76is also curved. When the V-shaped sensor arm73′ is used, as indicated by a dotted line inFIG. 14, high resolution can be obtained during early periods of a stroke while resolution near the end of the stroke is somewhat compromised. When the curved sensor arm73″ is used, as indicated by a one-dot chain line inFIG. 14, a resolution obtained is intermediate between those of the sensor arms73and73′. However, the relationship between the angle of rotation and the stroke becomes linear, with the result that data processing using a sensor output can be easily conducted.

Hereinbelow, an operation of the master cylinder apparatus1is described. The master cylinder apparatus1is connected to a vehicle body using a stud bolt80. The stud bolt80extends from a front side of the flange portion10aof the cylinder body10through the cover70that accommodates the stroke sensor8.

First, when the BBW system is normally operated, the fail-safe valves2A and2B are closed. Therefore, dependent on a force applied to the brake pedal3, the primary piston11advances in a leftward direction as viewed inFIGS. 1 and 3, and a fluid pressure corresponding to the input from the brake pedal3is generated in the first fluid pressure chamber13. In this instance, the poppet valve54of the opening/closing means7is open due to engagement with the secondary piston12at the retracted position (FIG. 7). Therefore, the brake fluid in the first fluid pressure chamber13passes through the port50of the cylinder body10, the fluid passage51in the opening/closing means7and the fluid passage52in the simulator body40and is supplied to the pressure chamber S in the stroke simulator5.

Although the secondary piston12marginally advances in accordance with the rise in pressure in the first fluid pressure chamber13, the poppet valve54is not caused to close as a result of this movement of the secondary piston12.

When brake fluid is introduced into the pressure chamber S of the stroke simulator5, the piston46first moves downward against the spring force of the second spring49having a force smaller than that of the first spring47, so as to ensure an appropriate initial stroke of the brake pedal3. After the piston46abuts against the spring bearing48, the piston46moves downward against the spring force of the first spring47, which is larger than that of the second spring49, to thereby ensure a desired stroke of the brake pedal3. As the piston46moves downward, a reactive force acting on the brake pedal3increases. In this way, a so-called pedal resistance is generated, to thereby obtain an optimum pedal-feel. Meanwhile, the amount of movement of the primary piston11is monitored by the stroke sensor8. Based on a signal emitted from the stroke sensor8(a piston stroke), the electronic control unit in the BBW system controls the fluid pressure supplied to the wheel cylinders, to thereby obtain a desired braking force.

The braking force is controlled on the basis of a piston stroke in the above-mentioned manner. Thus, if a fluid is caused to repeatedly flow from behind the cup seal35to the first fluid pressure chamber13(a backside flow) under repetitive operation of the brake pedal3, a reactive force acting on the brake pedal3increases, which makes it difficult to obtain an appropriate braking force in correspondence with operation of the brake pedal. However, in this embodiment, as indicated inFIG. 5, the outer circumferential edge of the primary-side cup seal35abuts against the front wall surface38aof the annular groove38, thus preventing the backside flow. Therefore, there is no flow of fluid from behind the primary-side cup seal35to the first fluid pressure chamber13, so that a desired braking force can be stably obtained, even when the brake pedal3is repeatedly operated.

Next, description is made with regard to an operation of the master cylinder apparatus1in the event of failure of the BBW system. In this case, the fail-safe valves2A and2B are opened, and the master cylinder4is fluidly connected to the wheel cylinders. Then, the primary piston11advances in accordance with a force applied to the brake pedal3, so as to increase a fluid pressure in the first fluid pressure chamber13. The brake fluid in the first fluid pressure chamber13flows from the first discharge port16through the fail-safe valve2A to the corresponding wheel cylinders. On the other hand, the secondary piston12also advances according to an increase in the fluid pressure in the first fluid pressure chamber13, and brake fluid in the second fluid pressure chamber15is supplied from the second discharge port17through the fail-safe valve2B to the corresponding wheel cylinders.

As the secondary piston12advances, as indicated inFIG. 10, the rocking lever55of the opening/closing means7rocks about the shaft portion65, and the valve body62of the poppet valve54is seated on the valve seat61, to thereby close the fluid passage52(the simulator passage6) in the simulator body40.

Consequently, the supply of brake fluid to the stroke simulator5is stopped; and as a result, the master cylinder apparatus1operates as a manual brake, and supplies a desired amount of brake fluid to each of the wheel cylinders.

In this state, the fluid pressure in the first fluid pressure chamber13acts on the rear side of the valve body62of the poppet valve54. Therefore, the poppet valve54reliably closes the simulator passage6, with the aid of the elastic member64connected to the lower end thereof. When the BBW system is normally operated, the valve body62of the poppet valve54, which is in a standby condition, is suspended at a position separate from the valve seat61. Therefore, if the valve body62is placed in a standby condition for a prolonged period of time, the elastic member64will not be subject to deformation or damage, and the simulator passage6can consequently be reliably closed in the event of failure of the system.

When the brake pedal3is released, since the spring force of the second return spring21is larger than that of the first return spring20, the secondary piston12is first retracted, thus lowering a fluid pressure in the second fluid pressure chamber15. Consequently, the brake fluid returns from the wheel cylinders to the second fluid pressure chamber15, while the brake fluid is supplied from the reservoir29through the cup seal37to the second fluid pressure chamber15. The secondary piston12finally abuts against the stopper pin25. Thus, a retracted position of the secondary piston12is limited, and the secondary piston12is stopped at its initial position. In this instance, the second fluid pressure chamber15and the reservoir29are communicated with each other through the supply port33formed in the cup-like portion12aof the secondary piston12, to thereby control the brake fluid in the second fluid pressure chamber15. On the other hand, under the spring force of the first return spring20, the primary piston11returns to its initial position later than the secondary piston12. Consequently, the first fluid pressure chamber13and the reservoir29are communicated with each other through the supply port32formed in the cup-like portion11aof the primary piston11, to thereby control the brake fluid in the first fluid pressure chamber13. As described above, the cup seal35on a side of the primary piston11is configured to prevent the backside flow (FIG. 5). Therefore, no brake fluid is supplied from the reservoir29to the first fluid pressure chamber13during a return stroke of the primary piston11. Thereafter, when the brake pedal3is operated, the primary piston11and the secondary piston12advance again. At this time, an invalid stroke is not generated, since the primary piston11and the secondary piston12are accurately returned to their initial positions by means of the piston guide23and the stopper pin25. Therefore, stable braking can be conducted, even in the event of failure of the BBW system.

In this embodiment, the opening/closing means7is provided in the simulator passage6directly extended from the cylinder body10of the master cylinder4to the stroke simulator5. Therefore, there is no need to provide an extra valve element inside the secondary piston12, thus achieving a reduction in size of the secondary piston12. In this embodiment, the opening/closing means7comprises the poppet valve54. Therefore, it is unnecessary to use a seal member, with concomitant risk of damage, and the opening/closing means7can be stably operated over a prolonged period of time. This markedly improves reliability of the apparatus in the event of failure of the BBW system. Further, the poppet valve54is opened and closed by the rocking lever55which moves together with the secondary piston12. Therefore, no special drive means is necessary for operating the poppet valve54, which simplifies a structure of the opening/closing means7. Further, in this embodiment, the stroke sensor8has a mechanism such that a linear motion of the primary piston11is converted to a rotational motion through engagement between the sensor pin75and the sensor arm73. Therefore, the stroke sensor8is made simple in structure and is reduced in size, which results in an overall reduction in size of the entire apparatus.

A structure of the opening/closing means7is not particularly limited. Instead of the poppet valve54in the above-mentioned embodiment, a tilt valve, a slide valve or a spool valve may be used.

In the above-mentioned embodiment, use is made of a master cylinder in which the second return spring21for the secondary piston12has a larger spring force than the first return spring20for the primary piston11. However, this does not limit the present invention. The master cylinder may be such that the first return spring20for the primary piston11has a larger spring force than the second return spring21for the secondary piston12. In this case, since the retracted position of the secondary piston12is limited by the stopper pin25, it is possible to avoid a phenomenon such that when the brake is released the secondary piston12travels an excessive distance, i.e., overshoot.

The entire disclosure of Japanese Patent Application No. 2003-341338 filed on Sep. 30, 2003 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.