JPH0615328B2 - Brake booster - Google Patents

Description

The present invention relates to a brake booster used in a vehicle.

[Prior Art] Generally, a brake booster includes a power piston slidably arranged in a shell, a valve mechanism housed in a valve body of a shaft portion of the power piston, and a power piston in a front side in an operating direction of the power piston. A constant pressure chamber formed and a variable pressure chamber formed on the rear side, and an input shaft for operating the valve plunger that constitutes the valve mechanism to switch the flow path and supplying pressure fluid to the variable pressure chamber to advance the power piston. Is equipped with.

[Problems to be Solved by the Invention] However, in the conventional brake booster of this type, the servo ratio is substantially constant and cannot be changed. Therefore, for example, when using the brake booster as a brake booster for trucks,
There is a difference in the driver's braking feeling between the empty state and the loaded state,
When the vehicle was loaded, it was often perceived that the braking force was small. Further, even in the same braking state, the driver may sense that the braking force is large or small, and there is an individual difference in the desired braking feeling.

[Means for Solving the Problems] In view of such circumstances, the present invention provides the valve plunger with a plurality of auxiliary pistons, and forms at least three pressure chambers defined by the auxiliary pistons in the valve body. , When the pressure is introduced into each pressure chamber, the pressure is applied to the auxiliary piston to urge the valve plunger in the forward direction or the backward direction of the power piston, and the pressure in at least two pressure chambers is controlled to be large or small. The flow path switching mechanism is provided.

[Operation] With such a configuration, by introducing the pressure fluid into each of the pressure chambers, the valve plunger can be urged in the forward direction or the backward direction via the auxiliary piston, so that the servo ratio is changed. can do.

Since at least three pressure chambers are provided and the pressure in at least two pressure chambers is controlled to be large or small by the flow path switching mechanism, the diameters of the plurality of auxiliary pistons are made different and are supplied to each pressure chamber. Multiple servo ratios can be obtained by varying the size of the pressure fluid.

[Embodiment] The present invention will be described below with reference to the illustrated embodiment. In FIG. 1, a power piston 2 is slidably provided in a shell 1 of a brake booster, and a diaphragm 3 is attached to a rear surface of the power piston 2. The power piston 2 and the diaphragm 3 divide the inside of the shell 1 into a constant pressure chamber 4 at the front and a variable pressure chamber 5 at the rear. A valve body 6 is integrally provided on the shaft portion of the power piston 2, and a valve mechanism 7 for switching the flow path in the valve body 6 is provided.
Is stored.

The valve mechanism 7 is the first valve seat 1 formed on the valve body 6.
0, a second valve seat 12 formed on the valve plunger 11 and both valve seats 10 and 12 are provided with a valve body 14 which is seated from the rear side of the power piston 2, that is, from the right side of FIG. 1 by the elastic force of a spring 13. There is. Then, the first valve seat 10 and the valve body 1
The outer side of the seat portion with 4 is communicated with the constant pressure chamber 4 through a passage 15 formed in the valve body 6, and the constant pressure chamber 4 is further provided with a negative pressure introducing pipe 16 provided in the shell 1 for intake of an engine (not shown). It is connected to a negative pressure source such as a manifold.

On the other hand, the first valve seat 10 and the valve body 14, and the second valve seat 1
2 and the valve body 14 have a valve body 6 at an intermediate portion between respective seal portions.
The variable pressure chamber 5 is communicated through the passage 17 formed in 1., and the inside of the seat portion of the second valve seat 12 and the valve body 14 is communicated with the atmosphere through the filter 18. The variable pressure chamber 5 is kept airtight from the outside by a seal member 19 slidably penetrating the valve body 6.

The valve plunger 11 which constitutes the valve mechanism 7 is connected to an input shaft 25 which is interlocked with a brake pedal (not shown), and the distal end face of the valve plunger 11 is housed in a recess formed in the base of the output shaft 26. Is facing. The output shaft 26 has a seal member 28.
Through, and is projected to the outside of the shell 1 and is interlocked with a piston of a master cylinder (not shown).

The power piston 2 and the valve body 6 are usually
The return spring 29 holds the valve in the non-actuated position, and in this non-actuated state, the key member 30 for preventing the valve plunger 11 from coming out of the valve body 6 comes into contact with the inner surface of the shell 1 and the valve body 6 is closed. The free rightward movement of the plunger 11 is regulated so that the switching operation of the fluid circuit by the valve mechanism 7 can be immediately obtained when the input shaft 25 and the valve plunger 11 are subsequently operated.

The above-mentioned configuration is basically the same as that of the conventionally known negative pressure type brake booster. When the brake pedal (not shown) is depressed and the input shaft 25 and the valve plunger 11 are moved leftward, the valve body 14 1st valve seat 1 of valve body 6
The valve body 14 is seated at 0 to shut off the communication between the variable pressure chamber 5 and the constant pressure chamber 4, and the valve body 14 serves as the second valve seat 1 of the valve plunger 11.
Since the atmosphere is communicated with the variable pressure chamber 5 by separating from 2, the atmospheric air is supplied into the variable pressure chamber 5 by this. When a pressure difference is generated before and after the power piston 2 and the power piston 2 is advanced against the repulsive force of the return spring 29, the braking action is performed.

When the depression force of the brake pedal is released from this brake operating state, the second valve seat 12 of the valve plunger 11 is seated on the valve body 14 to shut off the communication between the variable pressure chamber 5 and the atmosphere.
Since the valve body 14 is separated from the first valve seat 10 to communicate the variable pressure chamber 5 with the constant pressure chamber 4, the power piston 2 is returned to the original non-operating position by the return spring 29.

When the power piston 2 moves backward, the key member 3
When 0 abuts on the inner surface of the shell 1, the retreat of the valve plunger 11 interlocked with this stops, but the retreat of the power piston 2 and the valve body 6 continues, and the valve body 6
Of the valve body 6 causes the first valve seat 10 of the valve body 6 to
Valve body 6 that is close to 4 and the gap between them is almost zero
Comes into contact with the key member 30 and stops. Therefore, the flow path of the valve mechanism 7 is immediately switched when the input shaft 25 is advanced again.

However, in this embodiment, the valve body 6 is composed of a stepped tubular body 6a and a plate 6b attached to the left end face of the stepped tubular body 6 and a recess 3 is formed at the center of the left end of the stepped tubular body 6a.
5 is formed and the recess 35 is sealed by the plate 6b. A generally dish-shaped or two umbrella-shaped auxiliary pistons 36 are slidably fitted in the recesses 35 to form a step portion formed on the valve plunger 11 and the valve plunger 11 thereof.
The auxiliary piston 36 is attached to the valve plunger 11 by a snap ring 37 attached to the valve plunger 11.

In the present embodiment, the auxiliary piston 36 is composed of dish-shaped diaphragm plates 36a and 36b, respectively.
The outer diameter of the rear diaphragm plate 36a on the rear side is larger than the outer diameter of the front diaphragm plate 36b on the front side. And each diaphragm plate 36
diaphragms 38a, 38 on the back surfaces of a, 36b, respectively.
b is arranged, the rear diaphragm 38a is folded back to the outside and connected to the valve body 6, and the front diaphragm 38b is folded back to the inside and connected to the valve body 6.

Therefore, a pair of diaphragm plates 36a and 36b and a diaphragm 38 are provided in the valve body 6.
three pressure chambers 39R, 39C, 39 by a, 38b
F is formed. And the diaphragm 38
In consideration of the bending directions of a and 38b, fluid pressure is introduced into the pressure chamber 39R and the pressure chamber 39F on the front side of the auxiliary piston 36, and the auxiliary piston 36 is moved by the fluid pressure introduced into the pressure chamber 39R. The auxiliary piston 36 can be biased in the backward direction by the fluid pressure introduced into the pressure chamber 39F in the forward direction. The remaining pressure chamber 39C is always communicated with the constant pressure chamber 4, that is, the negative pressure source, through a passage 41 formed in the valve body 6.

The pressure chamber 39R on the rear side of the auxiliary piston 36 is connected to the proportion valve 45 via a passage 42R formed in the valve body and a flexible conduit 43R spirally arranged in the constant pressure chamber 4, and further to a conduit 46. The flow path switching mechanism 47 and the conduit 48 communicate with the variable pressure chamber 5. These passages constitute a first passage 49 that connects the pressure chamber 39R with the variable pressure chamber 5. On the other hand, the pressure chamber 39F on the front side of the auxiliary piston 36 is connected to the flow passage switching mechanism 47 via a passage 42F formed in the valve body and a flexible conduit 43F spirally arranged in the constant pressure chamber 4. is doing.

Further, the bypass conduit 5 bypassing the proportion valve 45 is provided between the conduit 43R and the flow path switching mechanism 47.
No. 0 is connected, and the passage 42R, the conduit 43R, the bypass conduit 50, the flow path switching mechanism 47 and the conduit 48 allow the pressure chamber 39R and the variable pressure chamber 5 to communicate with each other by bypassing the proportion valve 45. Are configured.

Further, the flow path switching mechanism 47 is capable of communicating with a negative pressure source such as the intake manifold of the engine (not shown) through the conduit 52 and also with the atmosphere through the port 53, which will be described in detail later. As described above, the flow path switching mechanism 47 is operated by the control device 54 including a microcomputer, and the pressure chamber 39R and the variable pressure chamber 5 are connected to the pressure chamber 39R in response to the detection signal from the vehicle weight sensor 55 for detecting the loading state of the vehicle. The first passage 49 or the second passage 5
1 to communicate with each other, or the pressure chamber 3
Negative pressure or atmosphere can be introduced into 9R.

As the vehicle weight sensor 55, one having an appropriate configuration such as one that detects a loading amount from a suspension stroke between a vehicle body and an axle or one that detects a loading amount from a load switch provided in a seat is used. it can.

Next, an example of the proportion valve 45 will be described with reference to FIG.
The housing 60 is provided with a first valve body 63 supported by two diaphragms 61 and 62 and movable in the axial direction. The two diaphragms 61 and 62 have different pressure receiving areas, and a pressure chamber 64 is provided outside the diaphragm 61 having a small pressure receiving area, and a negative pressure chamber 65 is provided between the two diaphragms 61 and 62. A control pressure chamber 66 is sequentially formed on the outside of the diaphragm 62 having a pressure receiving area.

Further, a second valve body 67 made of a diaphragm is provided outside the control pressure chamber 66 in the housing 60, and the side of the control pressure chamber 66 partitioned by the second valve body 67 is a pressure chamber 68.
A negative pressure chamber 69 is provided on the opposite side, and the pressure chamber 68 and the above-mentioned pressure chamber 64 are communicated with each other through a passage 70 formed in the housing 60.

The second valve body 67 is seated on a valve seat 72 normally formed in the housing 60 by a spring 71 housed in a negative pressure chamber 69, and the first valve body 63 is a pressure chamber opposite to the negative pressure chamber 69. The second valve element 67 seated on the valve seat 72 is seated by the spring 73 housed in the housing 64 and having a weaker repulsive force than the spring 71. Then, the negative pressure chamber 69 is communicated with the negative pressure chamber 65 between the two diaphragms 61 and 62 through the opening 74 formed in the second valve body 67 and the passage 75 formed in the first valve body 63, and Negative pressure chamber 69 through conduit 7
A negative pressure source such as the intake manifold of the engine (not shown) is communicated via 6.

On the other hand, the pressure chamber 64 formed on the outside of the diaphragm 61 having a small pressure receiving area communicates with the flow path switching mechanism 47 through the above-mentioned conduit 46, and further through the conduit 48, the variable pressure chamber 5.
It is possible to communicate within. Further, the control pressure chamber 66 formed outside the diaphragm 62 having a large pressure receiving area communicates with the pressure chamber 39R formed inside the valve body 6 via the conduit 43R described above.

In the proportion valve 45 having such a configuration, when the pressure fluid in the variable pressure chamber 5 is introduced into the pressure chamber 64 via the flow path switching mechanism 47 and the conduit 46, the pressure fluid is transferred to the diaphragm 61. Working, second
The first valve body 63 and the second valve body 67 are integrally moved upward in FIG. 2 against the elastic force of the spring 71 that urges the valve body 67.

Then, from the pressure chamber 64 to the pressure chamber 6 via the passage 70.
The pressure fluid introduced into the second valve body 8 and the second valve body 67 and the valve seat 7
2 and the housing 60 and the outer peripheral surface of the first valve body 63 to flow into the control pressure chamber 66 and exert the fluid pressure on the diaphragm 62 to move the first valve body 63 to the second position.
It will be biased downward in the figure.

At this time, since the two diaphragms 61 and 62 increase the pressure receiving area on the side of the diaphragm 62, the control pressure chamber 6
The pressure fluid introduced into 6 moves the first valve body 63 downward, and when the first valve body 63 moves downward and the second valve body 67 sits on the valve seat 72, control is performed from the pressure chamber 68. The introduction of the pressurized fluid into the pressure chamber 66 is stopped. Then, when the pressure in the control pressure chamber 66 becomes too high, the first valve body 63 comes to separate from the second valve body 67 seated on the valve seat 72, so that the pressure in the control pressure chamber 66 becomes The two valve body 67 escapes into the negative pressure chamber 69.

Therefore, the pressure in the control pressure chamber 66 is reduced with respect to the pressure introduced into the pressure chamber 64, that is, the pressure in the variable pressure chamber 5, while maintaining a constant proportional relationship based on the pressure receiving areas of the two diaphragms 61 and 62. Therefore, when the pressure fluid in the variable pressure chamber 5 is introduced into the pressure chamber 64 via the flow path switching mechanism 47 and the conduit 46, it is proportional to the fluid pressure in the variable pressure chamber 5. The reduced fluid pressure can be supplied into the pressure chamber 39R in the valve body 6.

In the brake booster having the above configuration, the servo ratio can be changed in three stages according to the load amount. In particular, in this embodiment, the pressure introduced into the rear pressure chamber 39R and the front pressure are increased. The pressure introduced into the chamber 39F can be appropriately set in accordance with the difference between the pressure receiving areas of the diaphragm plates 36a and 36b.

That is, FIG. 3 shows a case where the front diaphragm plate 36b is set to be considerably smaller than the rear diaphragm plate 36a, and FIG. 4 shows a case where the front diaphragm plate 36b is set to the front diaphragm plate 36a.
6 shows the pressure source supplied to each pressure chamber when 6b is set to be slightly smaller, that is, the communication destination of each pressure chamber.

First, when the load capacity of the vehicle is large, the control device 54 detects the state by the vehicle weight sensor 55 and switches the flow path of the flow path switching mechanism 47, and in the example shown in FIG. The pressure of the variable pressure chamber 5 is reduced by the proportion valve 45 and introduced into the rear pressure chamber 39R, and the pressure of the variable pressure chamber 5 is directly introduced into the front pressure chamber 36F.
Further, in the example shown in FIG. 4, the pressure of the variable pressure chamber 5 is simultaneously introduced as it is into the rear pressure chamber 39R and the front pressure chamber 39F.

That is, in the example of FIG. 4, the diaphragm plate 36a,
Since the pressure receiving area difference between 36b is small, the auxiliary piston 36 can be advanced by an appropriate forward force even if the same pressure is introduced, but in FIG. 3 the pressure receiving area difference is increased. Therefore, by reducing the pressure introduced into the rear pressure chamber 39R with the proportion valve 45,
It is designed to obtain an appropriate amount of forward force.

In any case, when the brake booster is operated, the advancing force also acts on the auxiliary piston 36, so that the valve plunger 11 and the input shaft 25 move forward from the auxiliary piston 36. As a result, the driver can obtain a strong braking force with a light pedaling force. Therefore, the servo ratio is large in this state.

Next, when the vehicle load becomes medium, the control device 5
4 detects the state by the vehicle weight sensor 55, and the third
Also in the case of FIG. 4 and FIG. 4, all the pressure chambers 39R, 39C, 39F are made to communicate with the negative pressure source by the flow path switching mechanism 47. In this state, a pressure difference does not occur before and after the auxiliary piston 36 during the operation of the brake booster, so that the driver receives the reaction force at the originally determined servo ratio as in the case without the auxiliary piston 36. As described above, the servo ratio at this time is set to a medium level.

Further, when the load capacity of the vehicle becomes small, the control device 54
Switches the flow path of the flow path switching mechanism 47, and in the example shown in FIG. 3, the pressure chamber 39R on the rear side is communicated with the negative pressure source, and the pressure of the variable pressure chamber 5 is directly introduced to the pressure chamber 39F on the front side. In the example shown in FIG. 4, as in the case of FIG. 3, the pressure in the variable pressure chamber 5 is introduced into the pressure chamber 39F on the front side by using the pressure chamber 39R on the rear side as a negative pressure source. The pressure of the variable pressure chamber 5 introduced into 39F is reduced by the proportion valve 45.

In this case, in the example of FIG. 4, since the pressure receiving area of the front diaphragm plate 36b is small, even if the pressure of the variable pressure chamber 5 is introduced as it is, the auxiliary piston 36 can be biased in the backward direction by an appropriate size. In the example of FIG. 3, since the pressure receiving area of the front diaphragm plate 36b is large, by reducing the pressure of the variable pressure chamber 5 with the proportion valve 45, an appropriate amount of backward biasing force can be obtained. is there.

In any case, since the auxiliary piston 36 is biased in the backward direction, the driver is required to have a relatively heavy pedaling force in order to obtain a constant output, so that an excessive braking force is generated. It prevents the generation of skids and the like.

Next, the above description is an operation description relating to the vehicle weight sensor 55 included in the control device 54.
Further includes a plurality of sensors and switches as described below. That is, the control device 54 can detect the speed of the vehicle based on a signal from the vehicle speed sensor 80 that detects the speed of the vehicle. At the same time as controlling the servo ratio according to the vehicle weight, the vehicle speed is further increased. The flow path switching mechanism 47 is switched and controlled so that the servo ratio becomes smaller as the servo ratio increases.

Further, the control device 54 includes a manually operated servo ratio changeover switch 81, and the control point of the flow path changeover mechanism 47 can be changed according to the changeover position of the changeover switch 81. ing.

Further, the control device 54 includes a stop brake device 82, and after the control device 54 detects the stopped state of the vehicle by the vehicle speed sensor 80, the brake lamp switch 83 indicates that the brake pedal is continuously depressed for a predetermined time. In the case of FIG. 3, the rear pressure chamber 39R is detected from the port 53 of the flow path switching mechanism 47 open to the atmosphere via the bypass conduit 50, the conduits 43R and 43R, and the passage 42R. Atmospheric pressure is introduced into the interior of the vehicle, thereby actuating the brake booster and stopping the vehicle.

On the other hand, when the control device 54 detects that the accelerator switch 85 for detecting the depression of the accelerator pedal or the manual stop brake release switch 86 is operated, the atmospheric pressure introduced into the rear pressure chamber 39R is detected. Discharge to release the operation of the brake booster.

The parking brake device 82 is provided with a parking brake switch 87 and a driver-side door switch 88 so that the driver does not leave the driver's seat when the vehicle is stopped. When the door is opened without applying the brake, the alarm device 89 can be activated by the control device 54. Alternatively, when a load switch is provided in the driver's seat as the vehicle weight sensor 55, the load switch may be used instead of the door switch 88, and a switch for seat belt non-wearing alarm may be provided to give an alarm. You may make it emit.

By the way, in this embodiment, the auxiliary piston 36 is moved in the forward direction by the fluid pressure introduced into the pressure chamber 39R,
Since the auxiliary pistons 36 can be respectively biased in the backward direction by the fluid pressure introduced into 9F, the rear diaphragm plate 36a which particularly forms the pressure chamber 39R is formed.
Can be freely set to the size required for the stop brake device.

More specifically, since it is necessary to apply the greatest forward force to the auxiliary piston 36 when the stop brake device is activated, the pressure chamber 39C that is always in communication with the negative pressure source for that purpose.
Except that the rear pressure chamber 39R needs to communicate with the atmosphere, and the front pressure chamber 39F needs to communicate with the negative pressure source (see the state of the stop brake in FIG. 3).

When the size of the rear diaphragm plate 36a is set so that a sufficient braking force is obtained in this state, and the size of the rear diaphragm plate 36a becomes larger than necessary, the proportion valve 45 changes the pressure. Even if the pressure in the chamber 5 is greatly reduced and supplied to the rear pressure chamber 39R, the servo ratio may become too large, and the brake feeling may be impaired during normal brake operation. Since the fluid pressure can be introduced to 39F to urge the auxiliary piston 36 rearward, it is possible to obtain the optimum servo ratio as the stop brake device without impairing the brake feeling.

Further, the control device 54 includes an ultrasonic sensor 90 for detecting the presence or absence of an obstacle.
0 operates only when the vehicle retreats, and when the distance between the vehicle and the obstacle becomes a predetermined distance or less, the port 5 of the flow path switching mechanism 47 is
Atmospheric pressure is introduced into the pressure chamber 39R from 3 and the brake booster is automatically operated to stop the vehicle.

At this time, by the stop brake release switch 86,
Alternatively, the operation of the brake booster may be released by a unique manual brake release switch (not shown). Further, when the speed of the vehicle is low, the ultrasonic sensor 90 may be operated even when the vehicle is moving forward, and the vehicle may be automatically stopped.

Although not shown in the figure, a manual brake switch is provided as necessary so that the flow passage switching mechanism 47 can be appropriately switched and controlled in accordance with the operation position of the manual brake switch by the hand, separately from the stepping operation of the brake pedal by the foot. Therefore, it is possible to control the output of the brake booster in three stages.

As described above, the brake booster according to the present invention can be used for various purposes, but if necessary, some of the above-mentioned functions may be omitted, or another appropriate function may be added. Of course, it is also good. In any case, the control device 54 and the flow path switching mechanism 47 corresponding to various uses or functions are required, but it is easy for those skilled in the art to obtain such a control device 54 and the flow path switching mechanism 47. Let's do it.

Since the auxiliary piston 36 has an umbrella shape in the above embodiment, the pressure receiving area can be increased without interfering with the passage 15 and the reaction disk 27 can be inserted inside the auxiliary piston 36. Therefore, the axial length of the valve body 6 can be shortened. Further, since the auxiliary piston 36 has an umbrella shape, it is more advantageous in strength than a flat plate shape against bending force due to a pressure difference, and the piston can be made thinner.

In addition, since the diaphragm is used as the sealing means in the above embodiment, it is possible to eliminate the sliding resistance and the hysteresis accompanying the elastic deformation of the O-ring, as compared with the case where a sliding seal such as an O-ring is used. There is an advantage that you can. However, if a sliding seal such as an O-ring is used, even if a pressure higher than that of the pressure chamber 39R is introduced into the pressure chamber 39C, the sealing means does not reverse like the diaphragm, so that the degree of freedom in design is increased. Can be increased.

Further, the servo ratio can be finely controlled even if two or more proportion valves having changed proportional constants are provided and control pressure is supplied from each proportion valve to the pressure chamber, and the proportional constant is stepped as a proportion valve. It is possible to finely control the servo ratio by using a conventionally known proportion valve that can be changed dynamically or continuously.

Further, as the proportion valve, not only the one for reducing the pressure of the variable pressure chamber 5 at a constant ratio as in the above embodiment, but also the one for increasing the pressure can be used. When it can be constructed in a small size, it can be incorporated in the valve body 6.

Furthermore, by making the flow path switching mechanism an appropriate configuration,
The pressure chamber 39R on the rear side of the auxiliary piston 36 and the pressure chamber 39F on the front side of the auxiliary piston 36 can be independently switched selectively and communicated with any one of the negative pressure source, the variable pressure chamber 5 and the proportion valve 45. . It is considered to be easy for a person skilled in the art to obtain such a flow path switching mechanism, and in that case, a maximum of 9 different servo ratios can be obtained, so that more precise servo ratio control can be performed. Become.

[Advantages of the Invention] As described above, according to the present invention, at least three pressure chambers partitioned by the auxiliary piston are formed in the valve body, and the pressure in at least two pressure chambers is reduced by the flow path switching mechanism. Since the control is performed, it is possible to obtain many servo ratios such as the optimum servo ratio according to the loading amount and the optimum servo ratio according to the vehicle speed.

[Brief description of drawings]

FIG. 1 is a systematic cross-sectional view showing an embodiment of the present invention, FIG. 2 is a cross-sectional view showing a specific structure of the proportion valve 45 shown in FIG. 1, and FIGS. 3 and 4 are pressure chambers. 39
It is a figure explaining the communication destination of R, 39C, and 39F. 1 ... Shell, 2 ... Power piston 4 ... Constant pressure chamber, 5 ... Transformer chamber 6 ... Valve body, 7 ... Valve mechanism 11 ... Valve plunger, 25 ... Input shaft 36 ... Auxiliary piston, 47 ... ... Flow path switching mechanism 39R, 39C, 39F ... pressure chamber 45 ... proportion valve, 49 ... first passage 51 ... second passage 54 ... control device

Claims (1)

[Claims] 1. A power piston slidably disposed in a shell, a valve mechanism housed in a valve body of a shaft portion of the power piston, a constant pressure chamber and a rear side formed on the front side in the operating direction of the power piston. In a brake booster including a variable pressure chamber formed in the above, and an input shaft for operating the valve plunger that constitutes the above valve mechanism to switch the flow path and supplying pressure fluid to the variable pressure chamber to advance the power piston. The valve plunger is provided with a plurality of auxiliary pistons, and at least three pressure chambers defined by the auxiliary pistons are formed in the valve body so that when the pressures are introduced into the respective pressure chambers, the pressures of the auxiliary pistons are increased. To bias the valve plunger in the forward or backward direction of the power piston, and further reduce the pressure in at least two pressure chambers. A brake booster having a flow path switching mechanism for controlling.