JPH0292766A - Fluid pressure booster - Google Patents

Abstract

PURPOSE:To enable braking force to be accurately detected for its intensity by pressurizing a brake fluid pressure chamber of a master cylinder by both a booster piston and an inner side piston further applying a pressure of oil in the brake fluid pressure chamber as the reaction force acting on a valve piston. CONSTITUTION:In a tandem type master cylinder, a cylinder main unit connects to its rear end part a fluid pressure booster B, and it fits a booster piston 27 to a cylinder hole 24 of a front part sleeve 21. Here the booster piston 27 forms a stepped cylinder hole consisting of front and rear part cylinder holes fitting an inner side piston 29. This inner side piston 29 is hollow formed, and its hollow part arranges a valve piston 31, input piston 33 of large diameter adapted to this valve piston 31, extension rod 34 connected to a front end of the inner side piston 29 and a reaction force mechanism, while an input lever 39, operated by a brake pedal, is connected to the input piston 33.

Description

DETAILED DESCRIPTION OF THE INVENTION A0 Object of the Invention (1) Industrial Application Field The present invention relates to a fluid pressure booster for boosting a master cylinder used in an automobile etc. by the fluid pressure of a fluid pressure source, and particularly relates to a fluid pressure booster for boosting a master cylinder used in an automobile etc. This relates to an improvement that enables operation by input only even in the event of a power source failure.

(2) Conventional fluid pressure boosters are known, for example, from Japanese Patent Application Laid-open No. 60-1074.
This is already known as disclosed in Japanese Patent No. 44.

The fluid pressure booster disclosed in the above publication advances the booster piston using the fluid pressure of the fluid pressure source during normal boost operation to generate hydraulic pressure in the brake hydraulic chamber of the master cylinder, and when the fluid pressure source fails, , an input is given to the booster piston via the input rod, and the booster piston is moved forward in the same way as in normal conditions, thereby generating hydraulic pressure in the brake hydraulic chamber.

(3) Problems to be Solved by the Invention As mentioned above, in a fluid pressure booster that operates the booster piston regardless of whether the fluid pressure source is normal or malfunctioning,
Booster piston design is difficult.

That is, if the booster piston is formed to have a large diameter in order to shorten the stroke of the input rod as much as possible when the fluid pressure source is normal,
In the event of a failure of the fluid pressure source, significant input is required to operate the master cylinder. On the other hand, if the booster piston is formed to have a small diameter, the stroke of the input rod becomes long even when the fluid pressure source is normal, which impairs the operational feeling.

The present invention has been made in view of the above circumstances, and is designed to operate the master cylinder with a relatively short stroke of the input rod when the fluid pressure source is normal, and to operate the master cylinder with a relatively small input when the fluid pressure source is malfunctioning. It is an object of the present invention to provide a fluid pressure booster capable of accurately sensing the actuation reaction force of a master cylinder.

B0 Structure of the Invention (1) Means for Solving the Problems In order to achieve the above object, the present invention provides an input fluid pressure chamber that is connected to the rear part of the cylinder body of a master cylinder and that is connected to a hydraulic power source inside, and an output fluid pressure chamber. a booster body having a fluid pressure chamber; a booster piston that is slidably fitted into the booster body so that its front end faces a braking hydraulic chamber of a master cylinder and its rear end faces the output fluid pressure chamber; , fitted into the booster piston with its front end facing the braking hydraulic chamber;
When the booster piston moves forward, it moves forward together with the booster piston, but it also moves forward independently.The inner piston is slidably fitted into the inner piston, and the flow path between the input and output fluid pressure chambers is created by moving back and forth with respect to the inner piston. The inner piston includes a valve piston that controls the opening and closing of the flow path between the output fluid pressure chamber and the fluid tank, and an input rod that is connected to the rear end of the piston. The first feature is that the facing reaction force fluid pressure chamber is provided in communication with the braking hydraulic pressure chamber. Note that as the fluid pressure, oil pressure, air pressure, etc. are used.

In addition to the above features, the present invention also provides a master cylinder that includes:
The cylinder body has a tandem type housing a master piston that divides its interior into two chambers, a front brake hydraulic chamber and a rear brake hydraulic chamber, with the front end of the booster piston facing the rear brake hydraulic chamber and the rear brake hydraulic chamber facing the rear brake hydraulic chamber. The inner piston is connected mechanically to the inner piston and the master piston only when the rear brake hydraulic chamber system fails, and a part of the operating reaction force of the master piston is mechanically connected to the valve piston. The second feature is that a reaction force mechanism is provided to transmit the force.

(2) Effect In the first feature, when the fluid pressure source is normal, if communication is established between the person and the output fluid pressure chamber by operating the valve piston using the input rod, the fluid pressure of the fluid pressure source is output from the input fluid pressure chamber. The booster piston moves forward in response to the fluid pressure transmitted to the fluid pressure chamber. At this time, since the booster piston is accompanied by the inner piston, the braking hydraulic chamber of the master cylinder is pressurized by both pistons. Therefore, the braking hydraulic chamber can be effectively pressurized with a relatively short stroke of the booster piston.

At this time, the hydraulic pressure in the brake hydraulic pressure chamber is transmitted to the reaction fluid pressure chamber and acts on the valve piston as a reaction force.

Furthermore, if the input rod is advanced when the fluid pressure source fails, the input applied to the input rod is directly transmitted to the inner piston, causing it to advance. At this time, the inner piston moves forward leaving the booster piston behind, so the braking hydraulic chamber of the master cylinder is pressurized only by the inner piston. As a result, the stroke of the inner piston increases, but
This results in a reduction in input to the input rod.

In the second feature, when the booster piston is actuated when the rear brake hydraulic chamber system of the Kundem type master cylinder fails, the inner piston, which moves forward together with the booster piston, mechanically pushes the master piston through the reaction force mechanism, and the Enables pressurization of the partial brake hydraulic chamber. At this time, part of the reaction force of the master piston is mechanically transmitted to the valve piston via the reaction force mechanism.

(3) Embodiment Hereinafter, one embodiment of the present invention will be explained with reference to the drawings. In FIG. is formed. The inside of this oil tank 2 is
The lower half is connected to the front oil reservoir 2 by the partition wall 2a. and rear oil sump 2□
It is divided into

The cylinder hole 3 of the cylinder body 1 has a small diameter hole 3a passing directly below the oil tank 2, and a large diameter hole 3b connected to the rear end of the small diameter hole 3a.
It is structured in stages. Inside the cylinder hole 3 is a small diameter hole 3.
The front brake hydraulic chamber 5.a is slidably fitted into the front brake hydraulic chamber 5. and a rear brake hydraulic chamber 5□, and these brake hydraulic chambers 51.5□ are connected to two independent brake hydraulic circuits 7t via output ports + and 6g.

A stop ring 8 that restricts the backward limit of the master piston 4 is locked on the inner wall of the rear end of the small diameter hole 3a, and a return spring 9 that biases the master piston 4 toward the backward limit is connected to the front brake hydraulic chamber 5.
It will be reduced to 1.

An annular replenishment oil chamber 10 is formed on the outer periphery of the master piston 4 and is in constant communication with the front brake hydraulic chamber 51. A type relief boat valve 11 is provided on the master piston 4. That is, the relief boat valve 11 communicates between the front braking oil pressure chamber 5I and the replenishment oil chamber 10 when the master piston 4 is at its retracting limit, but when the piston 4 is moving forward, both chambers 5
1. Cut off between 10 and 10.

The front end portion of the booster body 20 of the fluid pressure booster B of the present invention is fitted and connected to the outer periphery of the rear end portion of the cylinder body 1. The inner surface of the booster body 20 includes a front sleeve 21 that comes into contact with the rear end of the cylinder body 1;
a rear sleeve 2 that abuts the rear end via a spacer 23;
2 is fitted. The front sleeve 21 has a cylinder hole 24 that continues to the large diameter hole 3b of the cylinder body 1, and
It has an inward flange 21a at the rear end. Further, the rear sleeve 22 has a cylinder hole 26 having a slightly larger diameter than the inner hole 25 of the flange 21a.

A booster piston 27 whose front end faces the rear brake hydraulic chamber 5□ is slidably fitted into the cylinder hole 24 of the front sleeve 21, and the booster piston 27 abuts against the flange 21a. The retraction limit is now regulated.

The booster piston 27 is inserted into the small diameter hole 3a of the cylinder body 1.
The front cylinder hole 28a has approximately the same diameter as the inner hole 25 of the flange 21a and opens on the front surface of the piston 27, and the rear cylinder hole 28b has a smaller diameter and has approximately the same diameter as the inner hole 25 of the flange 21a and opens on the rear surface of the piston 27. It has a stepped cylinder hole, into which a stepped inner piston 29 is slidably fitted.

The inner piston 29 has a stepped portion 29a that is separably engaged with the stepped portion 27a of the booster piston 27, and a return spring 30 that urges the inner piston 29 in the backward direction is connected to the rear brake hydraulic chamber. It is placed in 5□.

As clearly shown in FIG. 2, the inner piston 29 slidably passes through the inner hole 25 of the flange 21a, loosely passes through the cylinder hole 26 of the rear sleeve 22, and further extends through the rear end wall of the booster body 20. The booster body 1 is slidably penetrated through the booster body 1, and its rear end protrudes sufficiently long to the rear of the booster main body 1.

This inner piston 29 is hollow, and has a valve piston 31 in the hollow part, a large-diameter input piston 33 that abuts the front end of the valve piston 31 via a buffer rubber 32, and an extension connected to the front end of the inner piston 29. A rod 34 and a reaction force mechanism 35 are provided. A stop ring 36 that restricts the backward limit of the input piston 33 is locked to the rear end of the inner piston 29, and a valve spring 37 that springs the extension rod 34 toward the stop ring 36 is housed in the inner piston 29. be done.

Further, an input rod 39 that is operated by a brake pedal is connected to the input piston 33.

Annular oil passages 40 and 41 are formed between the booster main body 20 and the front sleeve 21, and an annular oil passage 42 is formed between the booster main body 20 and the rear sleeve 22. The annular oil passage 40 at the front is connected to the rear oil reservoir 2□ via a low-pressure conduit 43, and the annular oil passage 41 at the center is connected to a hydraulic pump 45 (see Fig. 1) as a fluid pressure source via a high-pressure conduit 44. ), the rear annular oil passage 42 is connected via a second low-pressure conduit 46 to an oil tank 47 (
(see Figure 1).

Further, the central annular oil passage 41 connects the rear sleeve 22 and the inner piston 2 through the radial oil passage 48 of the spacer 23.
9 communicates with an input hydraulic pressure chamber 49, that is, an input fluid pressure chamber.

The input hydraulic pressure chamber 49 includes a front seal member 50 that is attached to the flange 21a and brings the inner lip into close contact with the outer circumferential surface of the inner piston 29, and a front seal member 50 that is attached to the inner piston 29 and brings the outer circumferential lip into close contact with the inner circumferential surface of the rear sleeve 22. and a rear seal member 51 that is brought into close contact with the rear seal member 51. Therefore, the outer circumferential lip of the rear seal member 51 has a larger diameter than the inner circumferential lip of the front seal member 50, so that an axial thrust due to the hydraulic pressure of the input hydraulic chamber 49 is generated between the two seal members 50 and 51. This thrust force acts on the inner piston 29 as a retraction force via the rear seal member 51.

The inner piston 29 is provided with a radial inlet port 52 connected to the input hydraulic pressure chamber 49 and a radial outlet port 53 connected to the annular oil passage 42 .

The valve piston 31 has a central land 55 as a large mouth valve that opens and closes the inlet boat 52 and a rear land 56 as an outlet valve that opens and closes the outlet boat 53. An annular groove 58 defined between these lands 55 and 56 defines the valve piston 3.
1 and a series of oil passages 59 provided in the inner piston 29, the booster piston 27 communicates with an output hydraulic chamber 60, that is, an output fluid pressure chamber, which the rear end face of the booster piston 27 faces.

A reaction hydraulic chamber 61, ie, a reaction fluid pressure chamber, is provided in the inner piston 29, and the front end surface of the valve piston 31 faces the reaction hydraulic chamber 61, and the liquid chamber 61 communicates with the rear brake hydraulic chamber 5□ via an oil passage 54. .

Furthermore, a radial relief boat 62 that opens into the reaction hydraulic pressure chamber 61 is bored in the inner piston 29 . This relief boat 62 includes a series of oil passages 63 provided in the front sleeve 21, the booster piston 27, and the valve piston 31.
It communicates with the annular oil passage 40 through the valve piston 31, and is opened and closed by a front land 57 serving as a relief port valve formed at the front of the valve piston 31.

The front land 57, center land 55 and rear land 56 of the valve piston 31 are connected to the relief port 62, the inlet port 5
2 and the exit boat 53 are arranged so that the opening and closing timings are as follows. That is, when the valve piston 31 is located at the retraction limit, the relief port 62 and the outlet port 5
3 is in the open state, and the inlet boat 52 is in the closed state. When the valve piston 31 advances from the retraction limit, the relief boat 62 is first closed by the front land 57, and then the outlet boat is closed by the rear land 56. 53 was closed,
After this exit port 53 is completely closed, the entrance boat 52 is opened by the central land 55.

Note that the relief port 62 and the exit boat 53 may be closed at the same time.

The reaction force machine tlI35 includes a collar 65, an elastic piston 66, and an output piston 67 that are fitted in order from the back into a bottomed cylinder hole 64 that opens on the front end surface of the inner piston 29. Cylinder hole 6
There will be no withdrawal from 4. A reaction force piston 69 facing the front end of the extension rod 34 is slidably fitted into the collar 65, and the reaction force piston 69 is formed to have a sufficiently smaller diameter than the elastic piston 66.

An output rod 70 is protruded from the front surface of the output piston 67, and the output rod 70 normally maintains a constant distance from the master piston 4.

Referring again to FIG. 1, the hydraulic pump 45 is connected to an electric motor 71 for driving it, and the high pressure conduit 44 is connected to an accumulator 72.

Next, the operation of this embodiment will be explained.

1 and 2 when the brake pedal 38 is inactive.
As shown in the figure, the valve piston 31 is held at the retracting limit by the valve spring 37, and the valve piston 31 opens the relief boat 62 and the outlet boat 53, and the inlet port 5
2 is closed.

Therefore, the rear brake hydraulic chamber 5□ of the master cylinder M communicates with the rear oil reservoir 2□ via the relief port 62 and the low pressure conduit 43 and is at atmospheric pressure, so the master piston 4 is moved back by the return spring 9. Retained for a limited time. In this state, the relief port valve 11 is open, so the front brake hydraulic chamber 5 also communicates with the front oil reservoir 21 and is at atmospheric pressure.

On the other hand, the output hydraulic pressure chamber 60 communicates with the oil tank 47 via the outlet boat 53 and the second low pressure conduit 46 and is at atmospheric pressure, so the booster piston 27 and the inner piston 29 are operated by the force of the return spring 30. It is held at the backward limit by the return force generated by the hydraulic pressure in the input hydraulic chamber 49.

During this time, pressure oil sent from the hydraulic pump 45 and the accumulator 72 through the high pressure conduit 44 is on standby in the input hydraulic chamber 49 .

Now, when the brake pedal 38 is depressed to brake the automobile, the valve piston 31 moves forward via the input rod 39 and the input piston 33, first the relief boat 62 is closed by the front land 57, and then the front land 57 closes, and then the front land 57 closes the relief boat 62, and then the front land 57 closes the relief boat 62. The rear land 56 closes the outlet port 53, and finally the central land 55 closes the outlet port 53.
Population boat 52 is opened.

Then, the pressure oil waiting in the input hydraulic chamber 49 enters the output hydraulic chamber 60 from the inlet port 52 through the annular groove 58.
The booster piston 27 moves forward in response to the oil pressure. At this time, the booster piston 27 and the inner piston 29 have their stepped portions 27a29a in contact with each other, so the booster piston 27 moves forward together with the inner piston 29.

As both pistons 27 and 29 move forward, since the relief boat 62 is already closed as described above, hydraulic pressure is generated in the rear brake hydraulic chamber 5□, and the master piston 4 moves forward in response to the hydraulic pressure. Hydraulic pressure is also generated in the front brake hydraulic chamber 51. As a result, hydraulic pressure is output from each of the hydraulic brakes 1.6□, and both brake hydraulic circuits 78 and 7□ can be operated simultaneously.

The advancement of the booster piston 27 and the inner piston 29 is
The center land 55 of the valve piston 31 connects the inlet boat 52
It stops when the is closed.

In other words, the booster piston 27 and the inner piston 29 follow the valve piston 31.

By the way, the booster piston 27 is formed to have a sufficiently larger diameter than the master piston 4, and according to such simultaneous operation of the booster piston 27 and the inner piston 29,
Hydraulic pressure can be effectively generated in the rear brake hydraulic chamber 5□ with a relatively short stroke. As a result, the stroke of the input rod 39, ie, the amount of depression of the brake pedal, is only required to be small, resulting in an improved operating feeling.

Further, since the relief boat 62 is closed by the front land 57 of the valve piston 31 before the booster piston 27 moves forward, the booster piston 27 can generate hydraulic pressure in the rear brake hydraulic chamber 5z at the same time as the booster piston 27 moves forward. There are almost no

Moreover, since the closing of the relief boat 62 by the front land 57 proceeds simultaneously with the closing of the outlet boat 53 by the rear land 56, the valve piston 31 and the input rod 39 have a special operating stroke for closing the relief boat 62. It will not increase. In particular, three lands 57, 55 .
Controlling the opening and closing of the relief boat 62, inlet boat 52, and outlet port 53 by means of the valve piston 56 is extremely effective in accurately obtaining the opening and closing timings of these with a short operating stroke of the valve piston 31.

During such normal braking, the output rod 70 does not come into contact with the master piston 4. and rear brake hydraulic chamber 5
．． The generated hydraulic pressure is also transmitted to the reaction force hydraulic chamber 61 through the oil passage 54, and acts on the front surface of the valve piston 31 to provide a reaction force.

Even if pressure oil is supplied to the output hydraulic chamber 60 in this way, the valve piston 31 will receive a reaction force after hydraulic pressure is generated in the rear brake hydraulic chamber 5□ due to the advance of the booster piston 27, and furthermore, the valve piston 31 will receive a reaction force. 3■ is not equipped with a high-pressure sealing member that would cause large frictional resistance, so the reaction force received can be smoothly transmitted to the input rod 39, and as a result, the operational reaction force of the master cylinder M is transferred to the brake pedal. Accurate feedback is provided to the driver over a period of three days, allowing him to accurately inform the driver of the strength of the braking force.

During such braking, if the front brake hydraulic chamber 5° system brake hydraulic circuit 7. If a failure occurs in the front brake hydraulic chamber 51, no hydraulic pressure will be generated in the front brake hydraulic chamber 51 even when the master piston 4 moves forward. The forward stroke of the booster piston 27 and the inner piston 29 until they come into contact with the front end wall of the brake piston 27 and the inner piston 29 becomes an invalid stroke, but hydraulic pressure can be generated in the rear brake hydraulic chamber 5□ by the subsequent forward movement of both pistons 27 and 29. .

The operational reaction force of the master cylinder M at this time is fed back to the input side via the oil pressure in the reaction oil pressure chamber, as in the normal state.

On the other hand, if a failure occurs only in the brake hydraulic circuit 7□ of the rear brake hydraulic chamber 5□ system, the rear brake hydraulic chamber 5
Since no hydraulic pressure is generated in □, the forward stroke of the booster piston 27 and the inner piston 29 until the output rod 70 comes into contact with the rear end of the master piston 4 becomes an invalid stroke, but the forward movement of both pistons 27 and 29 thereafter moves the master piston 4 forward, and can generate hydraulic pressure in the front brake hydraulic chamber 51. At this time, the actuation reaction force of the master cylinder M is transmitted from the output rod 70 to the elastic piston 66 via the output piston 67 and compressively deforms the histone 66, so that part of the compression force is transferred to the reaction force piston 69. to the extension rod 34 and the valve piston 31 and to the input side.

Next, assume that a defect occurs in the hydraulic path from the hydraulic pump 45 to the input hydraulic chamber 49. In this case, since the hydraulic pressure disappears from the input hydraulic pressure chamber 49, the valve piston 31 is advanced by depressing the brake pedal 38, and the inlet boat 5
2 is opened, the booster piston 27 cannot move forward.

Therefore, when the valve piston 31 moves forward by a predetermined amount relative to the inner piston 29, the input piston 33 comes into contact with the inner piston 29, so that the pedal force applied to the brake pedal 38 is transmitted to the inner piston 29, causing it to move forward, and the master cylinder M can be operated only by pedal force, that is, input.

In this case, since the booster piston 29 is left behind at the backward limit, only the inner piston 29 is used to control the rear brake hydraulic chamber 5.
□ will be pressurized. Therefore, in order to obtain the same braking force, both the booster piston 27 and the inner piston 29 are used in the rear brake hydraulic chamber 5. Although the stroke of the inner piston 29 increases compared to when the booster operates to pressurize the inner piston 29, the input can be reduced thereby.

Further, when only the inner piston 29 moves forward, its stepped portion 29a separates from the stepped portion 27a of the booster piston 27, and the volume between both stepped portions 27a and 29a increases, creating negative pressure there. , the above volume is extremely small, and due to the generation of negative pressure, the rear oil sump 2. Since the oil is sucked between the stepped portions 27a and 29a through the low pressure conduit 43, the annular oil passage 42, and the oil passage 63, the forward resistance exerted on the negative pressure inner piston 29 is small.

Furthermore, in this case, since the return force of the hydraulic pressure of the input hydraulic pressure chamber 49 to the inner piston 29 has also disappeared, the forward movement resistance of the inner piston 29 is also reduced.

C1 Effects of the Invention As described above, according to the first feature of the present invention, when the fluid pressure source is normal, the braking hydraulic chamber of the master cylinder is pressurized by both the booster piston and the inner piston, thereby preventing failure of the fluid pressure source. Since the brake hydraulic pressure chamber is sometimes pressurized only by the inner piston, the stroke of the input rod can be shortened during normal conditions to improve the operating feeling, and the input can be reduced in the event of failure.

Moreover, when the fluid pressure source is normal, the hydraulic pressure in the brake hydraulic chamber generated by the operation of the booster piston acts on the valve piston as a reaction force, so the operator can accurately determine the strength of the master cylinder's operation reaction force and therefore the braking force. can be sensed.

According to a second feature of the present invention, the master cylinder is
The cylinder body has a tandem type housing a master piston that divides its interior into two chambers, a front brake hydraulic chamber and a rear brake hydraulic chamber, with the front end of the booster piston facing the rear brake hydraulic chamber and the rear brake hydraulic chamber facing the rear brake hydraulic chamber. The inner piston is connected mechanically to the inner piston and the master piston only when the rear brake hydraulic chamber system fails, and a part of the operating reaction force of the master piston is transferred to the valve piston. Even if the rear brake hydraulic chamber system fails, the master piston is mechanically pushed through the inner piston and the reaction force mechanism by the operation of the booster piston, and the front brake is maintained. The hydraulic chamber can be pressurized. Furthermore, since a portion of the reaction force of the master piston at this time is transmitted to the valve piston via the reaction force mechanism, the operator can accurately sense the strength of the braking force in this case as well.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention, and FIG. 1 is a longitudinal sectional side view of a fluid pressure booster of the present invention coupled to a tandem master cylinder, and FIG. 2 is an enlarged view of the main parts of FIG. 1. . B...Fluid pressure booster, M...Tandem type master cylinder...Cylinder body, 51...Front braking hydraulic chamber, 52
...Brake hydraulic chamber, 20...Booster body, 27...
・Booster piston, 29...inner piston, 31・
...Valve piston, 39...Input rod, 45...Hydraulic pump as fluid pressure source, 47...Oil tank as fluid tank, 49...Input hydraulic chamber as input fluid pressure chamber, 52 ...Entrance boat, 53...Exit boat 60.
... Output hydraulic chamber as an output fluid pressure chamber, 61... Reaction hydraulic chamber as a reaction fluid pressure chamber

Claims (2)

[Claims] (1) Continuously installed at the rear of the cylinder body of the master cylinder,
The booster body has an input fluid pressure chamber connected to a fluid pressure source and an output fluid pressure chamber therein, and a booster body that is slidably fitted into the booster body so that the front end faces the braking hydraulic chamber of the master cylinder, and a rear end. a booster piston having an end facing the output fluid pressure chamber; and an inner piston having a front end facing the braking hydraulic pressure chamber and fitting into the booster piston, and moving forward together with the booster piston when the booster piston moves forward, but capable of moving forward independently. , is slidably fitted into the inner piston, and controls the opening and closing of the flow path between the input and output fluid pressure chambers by moving forward and backward with respect to the inner piston, and the opening and closing of the flow path between the output fluid pressure chamber and the fluid tank. The valve piston is provided with a control valve piston and an input rod connected to the rear end of the piston, and the inner piston is provided with a reaction fluid pressure chamber facing the front end of the valve piston and communicating with the braking hydraulic pressure chamber. A fluid pressure booster featuring: (2) In the item described in paragraph (1), the master cylinder is configured in a tandem type in which a master piston is housed in the cylinder body, the interior of which is divided into two chambers: a front brake hydraulic pressure chamber and a rear brake hydraulic pressure chamber. The rear brake hydraulic pressure chamber faces the front end of the booster piston and communicates with the reaction fluid pressure chamber, and the inner piston is mechanically connected between the inner piston and the master piston only when the rear brake hydraulic chamber system fails. A fluid pressure booster characterized by being provided with a reaction force mechanism that is connected to the valve piston and mechanically transmits a part of the actuation reaction force of the master piston to the valve piston.