JP2906763B2 - Vehicle brake system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a brake system for a vehicle, and more particularly to a hydraulic system for boosting a master cylinder in response to a brake operating member by using a power hydraulic pressure output from a power hydraulic pressure source as a boost pressure. The present invention relates to a vehicle brake device including a booster.

[0002]

2. Description of the Related Art In recent years, the mounting space for a brake system has become extremely narrow due to the size and design of engines and suspension systems. Therefore, it is necessary to shorten the overall length of the master cylinder. For this reason, a method of shortening a tandem type master cylinder in which two master pistons are arranged in series has been considered.

[0003] For example, in the device disclosed in Japanese Patent Application Laid-Open No. 2-41966, an auxiliary cylinder is interposed in a hydraulic path that connects and connects a rear wheel cylinder and a hydraulic booster.
One of the two chambers formed in the auxiliary cylinder is connected to the rear wheel cylinder via a piston which is slidably fitted in the auxiliary cylinder in a liquid-tight manner, and the other is connected to the hydraulic booster. ing. In the above configuration, when the brake operation member is operated, the output hydraulic pressure of the hydraulic booster is applied to one chamber of the auxiliary cylinder. As a result, the piston in the auxiliary cylinder slides to reduce the size of the other chamber, and a hydraulic pressure corresponding to the output hydraulic pressure of the hydraulic booster is applied to the rear wheel cylinder connected to the other chamber. You. Therefore, it is not necessary to generate the brake fluid pressure applied to the rear wheel cylinder by the master cylinder, and the total length of the master cylinder can be reduced by reducing one master piston.

[0004]

However, in the device disclosed in the above publication, if the power hydraulic pressure source fails and the hydraulic booster cannot generate boost pressure, one of the auxiliary cylinders connected to the hydraulic booster is connected. The output hydraulic pressure from the hydraulic booster is not applied to the chamber. At this time, the piston in the auxiliary cylinder does not slide, and no brake fluid pressure is applied to the rear wheel cylinder connected to the other chamber of the auxiliary cylinder. Therefore, the brake on the rear wheel does not work,
There is a problem that it is necessary to rely on a normal front wheel side brake to obtain a braking force.

Accordingly, the present invention has been made in view of the above-mentioned problems, and achieves the initial object of shortening the overall length of the master cylinder while maintaining all wheels even when the hydraulic booster cannot generate boost pressure. It is an object of the present invention to provide a vehicle brake device that can secure a braking force.

[0006]

In order to achieve the above object, a vehicle brake system according to the present invention comprises a master cylinder which generates a brake fluid pressure in response to a brake operating member, and a brake fluid which is pressurized to a predetermined pressure. A power hydraulic pressure source that outputs a power hydraulic pressure, and a hydraulic booster that boosts and drives the master cylinder in response to the brake operating member with the output power hydraulic pressure of the power hydraulic pressure source as a boost source. A first wheel cylinder connected to the master cylinder via a hydraulic path, and a second wheel cylinder connected to the hydraulic booster via a hydraulic path.
A brake cylinder for a vehicle, comprising: an auxiliary cylinder disposed in the hydraulic path connected to the hydraulic booster and the second wheel cylinder, wherein the auxiliary cylinder slides in the auxiliary cylinder. A first movable piston and a second movable piston are fitted face-to-face, and the first of the three chambers formed in the cylinder by the first piston and the second piston; A first chamber formed between a piston and the second piston is connected to the second wheel cylinder, and a second chamber, one of the remaining two chambers, is connected to the master cylinder while the other is connected to the master cylinder. The point is that a certain third chamber is connected to the hydraulic booster.

[0007]

With the above construction, in the vehicle brake device of the present invention, when the brake operating member is operated, the hydraulic booster inputs the output power hydraulic pressure of the power hydraulic pressure source, and the master cylinder is operated in accordance with the operation of the brake operating member. The booster is driven and the output hydraulic pressure is applied to the second chamber of the auxiliary cylinder. By the boost driving of the master cylinder, the brake fluid pressure of the master cylinder is applied to the first chamber and the third chamber of the auxiliary cylinder connected to the master cylinder. Normally, since the output hydraulic pressure from the hydraulic booster is higher than the brake hydraulic pressure of the master cylinder, the first piston is stopped without sliding due to the urging force of the elastic member, and only the second piston is stopped by the elastic member. Slides in the opposite direction to the biasing force of. Then, the volume between the two pistons in the auxiliary cylinder is reduced, and a brake hydraulic pressure that matches the output hydraulic pressure from the hydraulic booster is applied to the second wheel cylinder connected to the auxiliary cylinder.

If the output hydraulic pressure from the hydraulic booster is not applied to the auxiliary cylinder, the second piston is stopped by the urging force of the elastic member, and the first piston is controlled by the brake hydraulic pressure from the master cylinder. It is applied and slides in the direction opposite to the biasing force of the elastic member. Then, the volume between the two pistons in the auxiliary cylinder is reduced, and a brake hydraulic pressure that matches the output hydraulic pressure from the master cylinder is applied to the second wheel cylinder connected to the auxiliary cylinder.

Therefore, even if the output hydraulic pressure from the hydraulic booster is not applied to the auxiliary cylinder, the brake hydraulic pressure can be generated in the auxiliary cylinder and applied to the second wheel cylinder, and the braking force is secured on all wheels. can do.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration diagram of an embodiment of the present invention.

The brake fluid pressure control device is a master cylinder 2
0, a hydraulic pressure booster 3 is provided, and a pedaling force applied to a brake pedal 1 as a brake operating member is applied to an input rod 21.
And the fluid input from the reservoir 51 in which the brake fluid in the atmospheric pressure is stored, or the power hydraulic pressure source 90 for increasing the pressure of the brake fluid stored in the reservoir 51 in accordance with the braking force. The pressure is appropriately controlled and output to front wheel cylinders 15 and 16 and rear wheel cylinders 17 and 18.

First, in the master cylinder 20, a first cylinder hole 36 and a second cylinder hole 39 are formed in a housing 35a.
5 is slidably fitted. The second cylinder hole 39 accommodates the spring 66 and the front end of the valve rod 65a that contacts the spring 66. The master piston 25 is formed with a small-diameter land portion and a large-diameter land portion, and the first cylinder hole 36 is also formed with a small-diameter portion and a large-diameter portion so as to be adapted to form a stepped hole. A liquid supply chamber 37 is formed between the small-diameter land portion and the large-diameter land portion of the master piston 25 in the large-diameter portion of the first cylinder hole 36, and the front end of the master piston 25 is formed in the small-diameter portion of the first cylinder hole 36. The pressure chamber 32 is formed between the front walls 38. The pressure chamber 32 communicates with the liquid passage 41 via a port 63, and the liquid supply chamber 37 communicates with the reservoir 51 via a port 64. The pressure chamber 32 and the liquid supply chamber 37 communicate with each other through a passage hole 52 provided in the master piston 25 and the hole 26. The valve rod 65a regulates the sliding of the master cylinder 25 and controls the communication between the pressure chamber 32 and the liquid supply chamber 37.
5b is mounted. The hole 26 has a valve rod 65a
A valve body 65b mounted on the rear end is movably housed facing the passage hole 52. Further, the first cylinder hole 36
A return spring 67 for urging the master piston 25 rearward is accommodated between the front wall 38 and the master piston 25 in the small diameter portion.

Thus, the brake fluid supplied from the reservoir 51 to the fluid supply chamber 37 via the port 64 is filled into the pressure chamber 32 via the communication hole 52 and the hole 26 in the master piston 25. From this state, when the master piston 25 is pressed forward by the operation of the brake pedal 1 and slides, the communication hole 52 is closed by the valve body 65b, and the pressure chamber 32 is closed except for the port 63, so that the master piston 25 is closed. The pressure of the brake fluid is increased with sliding. At this time, the brake fluid amount in the reservoir 51 is detected by the sensor 7.

The pressure booster chamber 31 of the hydraulic booster 3 is formed in the housing 35a, and the power piston 24 is slidably fitted in the first cylinder hole 36. A hole 24a is formed in the power piston 24, and a reaction force piston 30 is slidably received in the hole 24a.

In the radial direction of the reaction force piston 30, a long hole 30a having a long axis in the axial direction and a through hole 30b orthogonal to the long hole 30a are formed. A pin 24c fixed to the power piston 24 is inserted into the long hole 30a.
, At least the sliding of the reaction force piston 30 in the rearward direction is restricted. Normally, the reaction piston 30 is urged by a spring 34 mounted between the power piston 24 and the reaction piston 30 to a position where sliding is regulated by the pin 24c. A spherical head formed at the other end of the input rod 21 having one end connected to the brake pedal 1 is inserted into the power piston 30. A stopper 69 is attached to the input rod 21 in the radial direction. The stopper 69 regulates the forward sliding of the input rod 21 and receives a backward biasing force from a spring 70 attached to the housing 35a. In the radial direction of the power piston 24, the reaction piston 30 overlaps with the through hole 30b when the piston is at the rearmost position, and a larger through hole 24d is formed. The stroke amount of the brake pedal 1 is detected by a stroke sensor 2.

One end of a through-hole 30b of the reaction force piston 30 is axially attached to a housing 35a by a pin 71, and
The spherical head of the support lever 72 that swings within 1 is fitted. One head of the control lever 73 rotatably joined to each other by the support lever 72 and the pin 71 is fitted in the through hole 24 d of the power piston 24. On the other head of the control lever 73, a pin 7 of the support lever 72 is provided.
1 are formed with cam slots.

Accordingly, when the input rod 21 and the reaction force piston 30 slide forward with respect to the power piston 24 pressed toward the brake pedal 1, the support lever 72 rotates clockwise about the pin 71 as an axis. Power is applied.
At this time, since one head of the control lever 73 is held in the through hole 24d of the power piston 24, the other head of the control lever 73 rotates counterclockwise about the pin 71, and The hole is guided by the pin 71 and moves in the same direction as the sliding direction of the input rod 21 and the reaction force piston 30, and the movement distance until the rear end of the hole 30a of the reaction force piston 30 contacts the pin 24c. Is formed.

The master piston 2 is provided in the housing 35a.
A spool valve hole communicating with the pressure doubler chamber 31 is formed substantially parallel to 5, and a spool valve 23 functioning as a hydraulic pressure control valve is fitted in the spool valve hole. The spool valve 23 is provided with a double pressure chamber 31.
A three-port two-position valve for controlling the pressure of the brake fluid in the inside, the first input port side is connected to the reservoir 51 via the pressure reducing port 57, and the second input port side is connected to the pressure increasing port 58. It is connected to the fluid pressure path 59 through. The output port side can communicate with the pressure doubler chamber 31. A protrusion 23 formed at the rear of the spool valve 23
A spring 60 for urging the spool valve 23 rearward is housed between the housing a and the housing 35a. The control lever 73 is in contact with the rear end of the spool valve 23.

The auxiliary cylinder 91 is connected to the master cylinder 20
, A cylinder hole 33 formed in a housing 35a, and a first piston 27 and a second piston 28 sliding in the cylinder hole 33. The first piston 27 and the second piston The valve rod 29 and the contracted spring 74 are housed between the two.
In the normal state, the first piston 27 is formed on the front wall of the cylinder 33 by the urging force of the spring 74.
, And the second piston 28 is urged until it comes into contact with the rear convex portion 76 formed on the rear wall of the cylinder hole 33.

First piston 27 and second piston 28
A cup 77 as a seal member is mounted on the outer peripheral surface of the cylinder hole 33, and a first hydraulic chamber 78 is formed in the cylinder hole 33 by contacting the inner peripheral surface of the cylinder hole 33. A second hydraulic chamber 86 is provided between the first piston 27 and the front wall.
A third hydraulic chamber 87 is formed between the second piston 28 and the rear wall. The second hydraulic chamber 86 and the third hydraulic chamber 87 communicate with the liquid passages 41 and 50 formed in the housing 35a, respectively. The first hydraulic chamber 78 communicates with the hydraulic passage 47 through a port 85, and also communicates with the reservoir 5 through a liquid passage 55 and a passage hole 79 and a hole 80 provided in the second cylinder.
It communicates with 1. A valve body 81 mounted on the rear end of the valve rod 29 is movably accommodated in the hole 80 so as to face the passage hole 79. A spring 83 accommodated in a valve cylinder 82 attached to the first piston 27 and biasing the valve rod 29 rearward is in contact with the front end of the valve rod 29.

Thus, the brake fluid supplied from the reservoir 51 through the fluid passage 55 fills the pressure chamber 32 through the communication hole 52 and the hole 26 in the first piston 25. In this state, when the second piston 28 is pressed forward by the operation of the brake pedal 1 and slides, the communication hole 79 is closed by the valve body 81, and the first hydraulic chamber 78 is connected to the port 8.
Except for 5, it is in a sealed state, and the brake fluid is pressurized.

The power hydraulic pressure source 90 includes the accumulator 5 and the hydraulic pump 4 driven by an electric motor. The hydraulic pressure path on the input side is connected to the reservoir 51 via the port 56, and the hydraulic pressure path 59 on the output side. Are connected to the accumulator 5 via the check valve 61, and the power hydraulic pressure is supplied to the required portions via the accumulator 5. The power hydraulic pressure is maintained at a predetermined pressure by intermittently controlling the electric motor in response to a signal from the pressure sensor 6 by an ECU (not shown).

The switching valves 9 and 10 for anti-skid are solenoid valves having a three-port two-position structure. Usually, the switching valve 9 is located at a position where the hydraulic passage 42 and the hydraulic passage 43 are communicated.
Reference numeral 0 denotes a position where the hydraulic pressure path 47 communicates with the hydraulic pressure path 49.
At the time of anti-skid control, the switching valves 9 and 10 are excited, the switching valve 9 communicates with the hydraulic pressure path 46 and the hydraulic pressure path 43, and the switching valve 9 communicates with the hydraulic pressure path 48 and the hydraulic pressure path 49. Switch to the next position.

The switching valve 8 for traction is an electromagnetic valve having a three-port two-position structure, and is a control valve for performing traction control when a drive wheel (rear wheel in this embodiment) slips during starting / acceleration. It is. At the time of traction control, the anti-skid switching valve 10 is energized and switched, and the switching valve 8 is also energized and switched to cut off the communication between the hydraulic pressure passage 44 and the hydraulic pressure passage 49 and to connect the hydraulic pressure passage 45 to the hydraulic pressure passage 45. Pressure line 49
And communicate with. As a result, the rear wheel cylinder 1
A high-pressure brake fluid is supplied from the accumulator 5 to 7 and 18.

The anti-skid control valves 11, 12, 13,
14 is a wheel cylinder 15 during anti-kid control,
This is a known three-port three-position solenoid valve that is switched to control the brake pressures of 16, 17, and 18.

The hydraulic passage 42 connects the liquid passage 41 and the switching valve 9, and the hydraulic passage 43 connects the switching valve 9 and the anti-skid control valves 11, 12.
The hydraulic passage 44 connects the liquid passage 50 and the switching valves 8 and 9, the hydraulic passage 45 connects the liquid passage 59 and the switching valve 8, and the hydraulic passage 48 connects the switching valve 8.
The hydraulic pressure path 49 connects the switching valve 10 and the anti-skid control valves 13 and 14 via a proportioning and bypass valve 19. Downstream of the anti-skid control valves 11, 12, front wheel cylinders 15, 16 are connected to the anti-skid control valves 13, 14, and rear wheel cylinders 17, 18 are connected to the anti-skid control valves 13, 14, respectively.

Next, the operation of the embodiment constructed as described above will be described. When the brake pedal 1 is not operated, the pressure chamber 32 of the master cylinder 20 communicates with the liquid supply chamber 37 and communicates with the wheel cylinders 15, 16 and the reservoir 51, respectively. The pressure in the reservoir 51, that is, substantially under atmospheric pressure.

On the other hand, when the power hydraulic pressure source 90 is driven, the power hydraulic pressure is applied to the port 58, but since the port 58 is closed, the hydraulic booster 3 does not function. In this state, the pressure doubler chamber 31 of the hydraulic pressure booster 3 communicates with the reservoir 51 via the spool valve 23 and is substantially under the atmospheric pressure, so that the second hydraulic pressure chamber 86 of the auxiliary cylinder 91 which communicates via the liquid passage 50 is provided. Are also under substantially the same pressure. The first hydraulic chamber 78 is also substantially at atmospheric pressure because it communicates with the reservoir 51 via the liquid passage 55, the passage hole 79, and the hole 80. Further, the second hydraulic chamber 86 is also provided with the liquid supply chamber 37, the passage hole 52, the hole 26, and the pressure chamber 3.
2. Since it is in communication with the reservoir 51 via the liquid passage 41, it is substantially under atmospheric pressure. At this time, the first piston 27 and the second piston 28 are in contact with the front convex portion 75 and the second piston 28 is in contact with the rear convex portion 76 by the urging force of the spring 74.

When the pedaling force is applied to the brake pedal 1,
When the reaction force piston 30 is pushed through the input rod 21 and moves until it comes into contact with the pin 24c of the power piston 24, the control lever 73 rotates counterclockwise with respect to the support lever 72, and the head pushes the spool valve 23. I do. As a result, the power hydraulic pressure is introduced from the power pressure source 90 into the pressure doubler chamber 31 and presses the power piston 24 so that the master piston 2
5 and the input rod 21.
The reaction force is transmitted to the brake pedal 1 via the. At the same time, the third hydraulic chamber 87 of the auxiliary cylinder 91 is moved from the double pressure chamber 31 to the third hydraulic chamber 87.
Output hydraulic pressure. When the master piston 25 starts sliding with the power piston 30, the valve body 65b
As a result, the communication hole 52 is closed, and the brake fluid pressure is output to the second fluid pressure chamber 86 of the auxiliary cylinder 91 and the wheel cylinders 15 and 16 as the pressure chamber 32 is reduced. At this time, since the hydraulic pressure applied to the third hydraulic chamber 87 is slightly higher than the hydraulic pressure applied to the second hydraulic chamber 86, the first piston 27 is biased by the spring 74 as shown in FIG. As a result, only the second piston 28 slides forward in the cylinder hole 33. When the second piston 28 slides forward, the valve body 81 closes the passage hole 79 and the first piston 28 slides.
The hydraulic chamber 78 is closed except for the port 85, and the brake fluid is pressurized, and the brake fluid pressure is output to the wheel cylinders 17 and 18.

On the other hand, when an excessive braking force is applied to the friction coefficient between the road surface and the tire and the wheels tend to lock,
Anti-skid control is performed. At the time of the anti-skid control, the switching valves 9 and 10 are energized to perform switching, so that the double pressure chamber 31 is switched.
After the anti-skid control valves 11 to 14 are communicated with each other, the anti-skid control valves 11 to 14 are switched to control the brake fluid pressure of the wheel cylinders 15 to 18 in three modes of increasing, holding, and reducing pressure. A known anti-skid control is performed.

When an excessive driving force is applied to the friction coefficient between the road surface and the tire and a slip occurs on the wheels, traction control is performed. In the traction control, the switching valves 8 and 10 are excited to switch the rear wheel cylinder 1
7, 18 and the power hydraulic pressure source 90, and the power hydraulic pressure source 9
From 0, brake fluid pressure is applied to the rear wheel cylinders 17,18.

If the power hydraulic pressure source 90 fails and cannot be boosted in the booster chamber 31, the brake pedal 1
Is transmitted directly to the master piston 25, and the pressure chamber 3
2 generates brake fluid pressure. At this time, since the output hydraulic pressure from the hydraulic pressure booster 3 is not applied to the second piston 28 of the auxiliary cylinder 91, the second piston 28 is moved by the urging force of the spring 74 to the rear convex portion 76 as shown in FIG.
However, the first piston 27 slides backward when the brake fluid pressure generated in the pressure chamber 32 is applied. First
When the piston 27 slides forward, the valve body 81 moves into the passage hole 7.
The first hydraulic chamber 78 is closed except for the port 85 so that the brake fluid is pressurized and the brake fluid pressure is output to the wheel cylinders 17 and 18. The brake fluid pressure generated in the pressure chamber 32 is also applied to the wheel cylinders 15 and 16. Therefore, the braking force of the four wheels is sufficiently secured.

When fluid leaks occur in the wheel cylinders 17 and 18 of the rear wheels or the hydraulic passages 47 and 49, these and the brake fluid filled in the first hydraulic chamber 78 of the auxiliary cylinder 91 flow out. Although the second piston 28 may come into contact with the first piston 27, the pressure chamber 32 of the master cylinder 20 is shut off.
There is no danger of the brake fluid flowing out of the flow path from 2 to the power hydraulic pressure source 90. Therefore, the brake fluid pressure is applied from the pressure chamber 31 to the wheel cylinders 15 and 16 of the front wheels, and the braking force of the front wheels is secured.

When the front wheel cylinders 11 and 12 or the hydraulic passages 42 and 43 leak, they are filled in the second hydraulic chamber 86 and the double pressure chamber 32 of the auxiliary cylinder 91. Although the brake fluid may flow out and cause the master piston 25 to bottom out on the front wall 38, since the master piston 25 is isolated from the pressure-doubling chamber 31, the pressure-doubling chamber 3
There is no fear that the brake fluid in the flow path from 1 to the power hydraulic pressure source 90 flows out. Therefore, the hydraulic pressure from the pressure doubler chamber 31
The second piston 28 is applied to the piston 28 and slides forward. At this time, the first piston 27 is biased by the spring 74 and is in contact with the front convex portion 75. When the second piston 28 slides forward, the valve body 81 closes the passage hole 79 and the first hydraulic chamber 78 is closed except for the port 85, so that the brake fluid is pressurized, and the wheel cylinders 17 and The brake fluid pressure is output to 18 to secure the braking force of the rear wheels.

In this embodiment, the first piston 27 corresponds to the first piston, the second piston 28 corresponds to the second piston, the spring 74 corresponds to the elastic member,
The first hydraulic chamber corresponds to the first chamber, the second hydraulic chamber corresponds to the second chamber, and the third hydraulic chamber corresponds to the third chamber.

The vehicle brake device according to the present invention is not limited to the above-described embodiment, but may be modified as follows without departing from the gist thereof. The relationship between the front wheels and the rear wheels is reversed, and the wheel cylinders 15 and 16 of the front wheels are reversed.
May be connected to the hydraulic booster 3 via the auxiliary cylinder 91.

[0037]

As described in detail above, the vehicular brake system of the present invention is provided with an auxiliary cylinder in which three chambers are formed by the first piston and the second piston, and one system is provided in the auxiliary cylinder. Since the brake operation is performed by the brake fluid pressure generated in the chamber between the first piston and the second piston, the total length of the master cylinder can be reduced. Also, even when the hydraulic pressure booster cannot generate boost pressure, the brake hydraulic pressure generated in the master cylinder is reduced to the first level.
This applies the piston to the first piston and contracts the chamber formed between the first piston and the second piston to generate the brake fluid pressure, so that the braking force can be secured on all the wheels.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a configuration of an embodiment.

FIG. 2 is a sectional view showing the operation of the embodiment.

FIG. 3 is a sectional view showing the operation of the embodiment.

[Explanation of symbols]

 3 Hydraulic Booster 15 Wheel Cylinder 16 Wheel Cylinder 16 Wheel Cylinder 18 Wheel Cylinder 20 Master Cylinder 27 First Piston 28 Second Piston 47 Spring 78 First Hydraulic Chamber 86 Second Hydraulic Chamber 87 Third Hydraulic Chamber 90 Power Liquid Pressure source 91 Auxiliary cylinder

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-52-11375 (JP, A) JP-A-2-70568 (JP, A) JP-A-1-208261 (JP, A) JP-A-2- 237860 (JP, A) JP-A-63-212161 (JP, A) JP-A-61-205546 (JP, A) JP-A-61-171654 (JP, A) (58) Fields investigated (Int. ⁶ , DB name) B60T 13/12

Claims (1)

(57) [Claims] A master cylinder that generates a brake fluid pressure in response to a brake operating member; a power fluid pressure source that boosts the brake fluid to a predetermined pressure and outputs a power fluid pressure; A hydraulic booster that boosts and drives the master cylinder in response to the brake operating member using output power hydraulic pressure as a boost source; a first wheel cylinder connected to the master cylinder via a hydraulic path; A second wheel cylinder connected to a hydraulic booster via a hydraulic passage; and a brake device for a vehicle, comprising: an auxiliary cylinder in the hydraulic passage connected to the hydraulic booster and the second wheel cylinder. A first piston and a second piston slidable in the auxiliary cylinder are fitted to the auxiliary cylinder so as to face each other. A first chamber formed between the first piston and the second piston among three chambers formed in the cylinder by the second piston is connected to the second wheel cylinder, And a second chamber connected to the master cylinder and one of the remaining two chambers connected to the hydraulic booster.