JP4765231B2 - Negative pressure booster - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a negative pressure type booster for a vehicle, and more particularly to a negative pressure type booster capable of compensating for a shortage of a pedal depression force during emergency braking.
[0002]
[Prior art]
When an obstacle suddenly appears on the vehicle's track and the driver rushes and depresses the brake pedal, that is, during emergency braking, the pedaling force applied to the brake pedal is a brake that is large enough to lock the wheel. There is a recent analysis result that the brake force of the vehicle is often not sufficiently exhibited because it is considerably smaller than the pedaling force that generates the hydraulic pressure. Based on the results of this analysis, a device has been proposed that determines whether the brake is a normal brake or an emergency brake from the depression speed of the brake pedal, and automatically increases the brake fluid pressure to a level that reaches the wheel lock during an emergency brake. It is being put into practical use.
[0003]
Various apparatuses having various configurations have been proposed as such an apparatus, for example, one described in WO01 / 32488A1. This includes a housing having at least one pressure chamber formed therein, a movable wall that is installed inside the housing so as to be capable of moving forward and backward with respect to the housing, and that divides the pressure chamber into a constant pressure chamber and a variable pressure chamber. , Power piston coupled to the movable wall, Input rod installed inside the power piston so as to be able to move forward and backward, and slidable back and forth with respect to the power piston connected to the input rod And a plunger having an atmospheric valve seat formed at the rear end thereof, an output member for outputting a propulsion force of the power piston accompanying the movement of the movable wall, and a power piston. And a first negative pressure valve seat facing rearward, and a valve seat member that can move forward and backward relative to the power piston and is substantially concentric with the first negative pressure valve seat. A valve mechanism having a second negative pressure valve seat, a movable portion facing the first negative pressure valve seat, and a fixed portion that is airtightly fixed to the power piston; and an urging force for urging the valve seat member toward the movable portion. A driving means for moving the valve seat member toward the movable portion by the biasing force of the biasing member when the advancement amount of the plunger with respect to the power piston is equal to or greater than a predetermined value; A holding means for holding the valve seat member at a predetermined position in the power piston when the value is smaller than the value, and a key member that is inserted into the radial hole of the power piston and restricts the amount of movement of the plunger in the front-rear direction with respect to the power piston. Is a negative pressure type booster.
[0004]
When the amount of forward relative movement of the plunger with respect to the power piston exceeds a predetermined value, in other words, when the driver rushes and depresses the brake pedal, the forward speed of the plunger is greater than the forward speed of the power piston. The engagement between the means and the valve seat member is released, the valve seat member is retracted, and the distance between the rear end surface of the reaction disk and the front end surface of the valve seal portion of the valve mechanism is increased, thereby increasing the so-called jumping amount. Emergency braking characteristics.
[0005]
That is, the same result as when the driver depresses the brake pedal strongly is obtained by switching the input-output characteristic of the negative pressure booster from the normal brake characteristic to the emergency brake characteristic during emergency braking.
[0006]
[Problems to be solved by the invention]
In this conventional negative pressure booster, when the engagement between the holding means and the valve seat member is released and the valve seat member is retracted, the key member that retracts integrally with the valve seat member is the flange surface of the plunger. There is a risk that the driver will feel uncomfortable due to sudden contact with the vehicle and noise generated.
[0007]
The present invention provides a negative pressure type booster capable of moderately reducing the retreating speed of the valve seat member and suppressing the contact noise caused by the contact between the key member retreating integrally with the valve seat member and the flange surface of the plunger. The purpose is to do.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, as described in claim 1, a housing having at least one pressure chamber formed therein, and installed in the housing so as to be capable of moving forward and backward with respect to the housing. A movable wall that divides the pressure chamber into a constant pressure chamber and a variable pressure chamber, a power piston coupled to the movable wall, and an input rod installed inside the power piston so as to be capable of moving forward and backward with respect to the power piston. A plunger that is connected to the input rod and is slidably inserted in the power piston so as to be slidable in the front-rear direction, and an atmospheric valve seat is formed at the rear end, and for the movement of the movable wall An output member for outputting the propulsive force of the power piston to the outside of the device, and a first negative pressure valve seat disposed on the power piston and facing rearward Said power piston forward and and retractable relative to said first negative pressure valve seat coaxial with a second vacuum valve seat formed on inserted through the valve seat member to the plunger, opposite to the first negative pressure valve seat And a valve mechanism having a fixed portion that is airtightly fixed to the power piston, a first biasing member that biases the valve seat member toward the movable portion, and the power of the plunger. Driving means for retracting the valve seat member toward the movable portion by the biasing force of the first biasing member when the advancement amount with respect to the piston is equal to or greater than a predetermined value, and the advancement amount of the plunger with respect to the power piston is a predetermined value A holding means for holding the valve seat member in a predetermined position in the power piston when it is smaller, and a power pin of the plunger inserted through a radial hole of the power piston. Thereby restricting the longitudinal movement amount with respect ton, in the vacuum booster comprising a key member which is inserted through the valve seat member, is fitted inserted into the plunger, it is restricted to move in the forward retainer And a second urging member compressed by the retainer and the rear flange portion of the plunger and applied with a predetermined set load, and when the advance amount of the plunger with respect to the power piston becomes a predetermined value or more The urging force of the first urging member causes the valve seat member to retreat integrally with the key member, abuts the retainer before the key member abuts against the plunger, and further the second A negative pressure booster characterized in that it moves backward by a predetermined amount toward the movable portion against the biasing force of the biasing member.
[0009]
According to the first aspect of the present invention, the valve seat member moves backward against the urging force of the second urging member, so that the retreating speed of the valve seat member is moderately moderated and moves backward integrally with the valve seat member. The contact noise caused by the contact between the key member and the flange surface of the plunger is suppressed.
[0010]
The rear means the brake pedal side or the vehicle rear side with respect to the negative pressure booster, and the front means the master cylinder side or the vehicle front side with respect to the negative pressure booster.
[0011]
Next, as described in claim 2, the second urging member is disposed coaxially with the plunger and has a front end surface portion that abuts against a rear end surface of the valve seat member. A pressure booster is preferred.
[0012]
According to the invention of claim 2, the second urging member is arranged coaxially with the plunger, and the contact locus between the valve seat member arranged coaxially with the plunger and the front end surface portion of the second urging member is formed. It is coaxial with the valve seat member and is not biased in the radial direction, and no moment is generated in the valve seat member and the second urging member. Therefore, there is no increase in frictional force between the valve seat member and the inner peripheral surface of the power piston, and the valve seat member can be smoothly moved.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
[0014]
1 and 2 are sectional views of a negative pressure booster according to an embodiment of the present invention.
[0015]
In FIG. 1, a negative pressure booster 10 includes a housing 14 that includes a front shell 11, a rear shell 12, and a movable wall 20, and the inside of which is divided into a constant pressure chamber 23 and a variable pressure chamber 24 by the movable wall 20. . The movable wall 20 in the housing 14 includes a metal plate 18 and a rubber diaphragm 19, and is installed so as to be movable in the front-rear direction with respect to the housing 14.
[0016]
The bead portion on the outer peripheral edge of the diaphragm 19 is airtightly held between the folded portion provided on the outer peripheral edge of the rear shell 12 and the front shell 11. The bead portion on the inner peripheral edge of the diaphragm 19 is airtightly fixed together with the plate 18 in a groove provided on the outer periphery of the front flange portion of the power piston 22.
[0017]
The constant pressure chamber 23 communicates with an engine intake manifold (not shown) which is a negative pressure source, and is always kept at a negative pressure during engine operation. The variable pressure chamber 24 is communicated / blocked with the constant pressure chamber 23 via the passage 60 and the valve mechanism 36, and is also communicated / blocked with the atmosphere via the valve mechanism 36.
[0018]
As shown in FIGS. 1 and 2, the power piston 22 has a first space 22 a located in the front (left side in FIGS. 1 and 2) and communicating with the constant pressure chamber, and a rear in the interior. It has the 2nd space 22b which is located (right in FIG.1 and FIG.2) and connects to air | atmosphere, and the intermediate part 221 interposed between the 1st space 22a and the 2nd space 22b. The intermediate portion 221 communicates the first space 22a and the second space 22b separately from the third space 22c and the third space 22c that communicates the first space 22a and the second space 22b at the center of the intermediate portion 221. And a passage 60 to be connected.
[0019]
Inside the power piston 22, an input rod 28 and a plunger 37 constituting the input member 27 penetrate the first space 22 a, the second space 22 b, and the third space 22 c and can move in the front-rear direction with respect to the power piston 22. It is installed as follows. Further, the input rod 28 and the plunger 37 are integrally coupled by a coupling portion 80, and the input rod 28 is coupled to a brake pedal (not shown) at the rear thereof.
[0020]
An output member 55 is fitted to the front end portion of the power piston 22, and a reaction force member 54 is disposed between the front end portion of the power piston 22 and the output member 55. Further, a contact member 70 is disposed between the reaction force member 54 and the front end portion of the plunger 37, and the contact member 70 is integrated with the power piston 22 so as to be movable in the front-rear direction. Is installed inside the locking member 50.
[0021]
In order to define the amount of movement of the plunger 37 and the input rod 28 with respect to the power piston 22, the key member 32 is inserted into the radial hole 33 formed in the intermediate portion 221 of the power piston 22. The thickness dimension of the key member 32 in the front-rear direction (left-right direction in FIGS. 1 and 2) is smaller than the dimension of the radial hole 33 in the front-rear direction. It can move in the front-rear direction by the indicated distance C.
[0022]
The key member 32 can come into contact with the rear shell 12 at the rear surfaces of both ends protruding to the outer peripheral side of the power piston 22, and the limit of movement of the power piston 22 to the rear with respect to the housing 14 is as shown in FIG. In addition, the front wall of the radial hole 33 is in contact with the front surface of the key member 32, and the rear surfaces of both ends of the key member 32 are in contact with the rear shell 12. The central portion of the key member 32 is disposed between the rear end surface 34 b of the first flange portion 34 formed at the central portion of the plunger 37 and the front end surface 35 a of the second flange portion 35. The rearward movement limit of the plunger 37 is a position where the rear end surface 34 b abuts on the front surface of the key member 32 and the rear surface of the key member 32 abuts on the rear wall of the radial hole 33. The forward movement limit of the plunger 37 with respect to the power piston 22 is such that the front end surface 35 a is in contact with the rear surface of the key member 32 and the front surface of the key member 32 is in contact with the front wall of the radial hole 33. is there.
[0023]
Inside the power piston 22, the variable pressure chamber 24 communicates with the constant pressure chamber 23 in accordance with the longitudinal movement of the plunger 37 relative to the power piston 22, and the variable pressure chamber 24 is connected to the constant pressure chamber 23. In addition, a valve mechanism 36 is provided for switching between an output holding action state that is cut off from the atmosphere and an output increase action state in which the variable pressure chamber 24 is cut off from the constant pressure chamber 23 and communicated with the atmosphere.
[0024]
The valve mechanism 36 includes an atmospheric valve seat 37 a formed integrally with the rear end of the plunger 37, a first negative pressure valve seat 38 formed integrally with the power piston 22, and a rear end of the valve seat member 40. The power piston 22 includes a second negative pressure valve seat 39 formed integrally with the movable portion 41, a movable portion 41 having valve seal portions 41a and 41b facing the first negative pressure valve seat 38 and the atmospheric valve seat 37a, respectively, and a retainer 42. The main component is a fixed portion 43 that is airtightly fixed to the tube and a cylindrical member 45 in which the movable portion 41 is urged forward by a spring 44.
[0025]
As shown in a perspective view in FIG. 3, the valve seat member 40 has a slit 40 b into which the first flange portion 34 protruding in the radial direction is inserted into the outer periphery of the plunger 37, and a penetration that penetrates the valve seat member 40 in the front-rear direction. A hole 40e is provided. Further, the key member 32 is inserted through the slit 40b so as to straddle the connecting portion 40a. The valve seat member 40 disposed in the third space 22 c of the intermediate portion 221 is movable in the front-rear direction with respect to the power piston 22.
[0026]
By inserting the first flange portion 34 of the plunger 37 into the through hole 40e of the valve seat member 40, the first flange portion 34 protrudes to the outside of the valve seat member 40 through the slit 40b. When the plunger 37 provided with the valve seat member 40 is disposed in the power piston 22, the first flange portion 34 comes into contact with the inner peripheral surface 221 a of the intermediate portion 221, and the plunger 37 is connected to the first flange portion. 34 is supported by the inner peripheral surface 221a of the intermediate portion 221 so as to be slidable.
[0027]
On the other hand, the valve seat member 40 is located between the outer peripheral portion of the plunger 37 and the inner peripheral portion of the intermediate portion 221 of the power piston 22, and the intermediate portion 221 at the rear end where the second negative pressure valve seat 39 is formed. It will contact | abut to the inner peripheral surface of this so that sliding is possible in the front-back direction. That is, the power piston 22 includes the intermediate portion 221 as a holding portion that holds the valve seat member 40 slidably rearward.
[0028]
On the outer periphery of the rear end portion of the valve seat member 40 inserted into the intermediate portion 221, an annular seal member 46 that keeps airtightness between the rear end portion of the valve seat member 40 and the inner peripheral surface of the intermediate portion 221 is mounted. Has been.
[0029]
A spring 59 disposed between a retainer 58 that is locked to the input rod 28 that is integrally connected to the plunger 37 and the coupling portion 80 and a retainer 42 that is locked to the power piston 22 is provided to the input rod 28. When the brake pedal is not depressed, the atmospheric valve seat 37a is brought into contact with the movable portion 41 of the valve mechanism 36 and the movable portion 41 is separated from the first negative pressure valve seat valve seat 38 by a distance A. Hold in a separated state.
[0030]
The passage 60 formed in the intermediate portion 221 and the first space 22a communicate the valve mechanism 36 and the constant pressure chamber 23, and the third space 22c communicates the valve mechanism 36 and the variable pressure chamber.
[0031]
Driving means for propelling the valve seat member 40 rearward is disposed in the front portion of the power piston 22, that is, in the intermediate portion 221. This driving means is composed of a spring 47 and a key member 32 that urge the valve seat member 40 rearward.
[0032]
A holding means 48 is disposed inside the front portion of the power piston 22, that is, in the first space 22a. The holding means 48 includes a holding member 52 having an engaging portion 52b that engages with the engaged portion 40c of the valve seat member 40, and a ring-shaped elastic body 53 that urges the holding member 52 in the direction of reducing the diameter. The locking portion 52 c of the holding member 52 is fitted in a locking groove 50 a of the locking member 50 fixed to the front portion of the power piston 22. For this reason, the holding member 52 cannot move in the front-rear direction, but is allowed to move in the radial direction around the locking portion 52c. As shown in FIG. 4, the holding member 52 has a three-divided hollow truncated cone shape obtained by cutting the hollow truncated cone in three planes passing through its central axis. The power piston 22 is assembled to face each other.
[0033]
In this embodiment, the retainer 61 shown in FIG. 5 and FIG. 6 is inserted into the outer periphery of the rear cylindrical portion of the plunger 37 and is snapped on an annular groove provided on the outer periphery of the rear cylindrical portion of the plunger 37. The ring 63 restricts the forward movement of the retainer 61. The spring 62 is compressed and disposed by the retainer 61 and the rear flange portion 37b of the plunger 37 so as to have a predetermined set load. The set load of the spring 62 is set smaller than the urging force of the spring 47 to the rear.
[0034]
As shown in FIGS. 5 and 6, the retainer 61 is inserted into the outer periphery of the rear cylindrical portion of the plunger 37 and has four claw portions 61 a whose front end surface 61 aa abuts against the snap ring 63 and a rear end surface 61 bb. It consists of a ring portion 61b that contacts the front end surface portion of the spring 62, and a connecting portion 61c that connects the claw portion 61a and the ring portion 61b.
[0035]
As will be described later, when the amount of forward relative movement of the plunger 37 with respect to the power piston 22 exceeds a predetermined value, the engagement between the holding means 48 and the valve seat member 40 is released, and an emergency brake operation is performed. At this time, the valve seat member 40 is retracted integrally with the key member 32 by the urging force of the spring 47, but before the rear surface of the key member 32 comes into contact with the front end surface 35a of the second flange portion 35 of the plunger 37, The rear end surface 40 h of the valve seat member 40 abuts on the front end surface 61 ba of the ring portion 61 b of the retainer 61, and further retracts while compressing the spring 62. When the rear surface of the key member 32 comes into contact with the front end surface 35a of the second flange portion 35 of the plunger 37, the backward movement of the valve seat member 40 is stopped.
[0036]
That is, since the valve seat member 40 is retracted against the urging force of the spring 62, the retracting speed of the valve seat member 40 is moderately moderated, and the key member 32 that retracts integrally with the valve seat member 40 is The contact sound at the time of contacting the front end surface 35a of the second flange portion 35 is suppressed.
[0037]
In the present embodiment, the spring 62 is restricted from moving in the radial direction by the inclined connecting portion 61c, and the inclined surface portion 37c provided between the rear cylindrical portion of the plunger 37 and the rear flange portion 37b. Is restricted from moving in the radial direction. That is, the spring 62 is arranged coaxially with the retainer 61, the plunger 37, the valve seat member 40, the power piston 22, and the like. Therefore, the contact locus between the valve seat member 40 and the retainer 61, in other words, the contact locus between the valve seat member 40 and the front end surface portion of the spring 62 is coaxial with the valve seat member 40 and is not biased in the radial direction. No moment is generated in the valve seat member 40 and the spring 62. For this reason, there is no increase in frictional force between the valve seat member 40 and the inner peripheral surface of the power piston 22, and the valve seat member 40 can move smoothly.
[0038]
The inner diameter E of the claw portion 61 a of the retainer 61 is smaller than the outer diameter of the second flange portion 35 of the plunger 37, and the inner diameter F of the ring portion 61 b is set larger than the outer diameter of the second flange portion 35. Further, the second flange portion 35 is provided with a plurality of recesses, and if the retainer 61 is moved rearward from the first flange portion 34 side of the plunger 37 with the recesses and the claw portions 61a corresponding to each other, The retainer 61 can be fitted on the outer periphery of the rear cylindrical portion of the plunger 37 through the two flange portions 35.
[0039]
Next, the operation of the negative pressure type booster of the present invention will be described. Since the operation as a booster in the normal operation state is generally known, the description thereof will be omitted and mainly during emergency braking. Will be described.
[0040]
The emergency brake characteristic of the negative pressure booster of the present invention is achieved by changing the jumping characteristic described above and applying a larger driving force to the output member than during normal braking. In order to change the jumping characteristics, the gap D between the contact member 70 and the reaction force member 54 in FIG. Enlarging the gap D is the same as increasing the distance between the contact surface 22d of the power piston 22 against the reaction force member 54 and the valve seal portion 41b. That is, the gap D is expanded by moving the valve seal portion 41b rearward, the output until the contact member 70 receives the reaction force from the reaction force member 54 is increased, and the ratio of the output to the input becomes infinite. The output in the so-called jumping state is made larger than that in the normal state.
[0041]
The actual operation will be described below.
[0042]
If the relative movement amount of the plunger 37 and the power piston 22 is greater than a predetermined value B during an emergency braking operation in which the driver rushes and depresses the brake pedal, the inclined surface 37b of the plunger 37 contacts the tapered portion 52a of the holding member 52. The holding member 52 urged in the direction of reducing the diameter by the ring-shaped elastic body 53 is expanded in the radial direction.
[0043]
When the minimum inner diameter portion 52aa of the tapered portion 52a rides on the first flange portion 34 of the plunger 37, the engagement between the engaged portion 40c of the valve seat member 40 and the engaging portion 52b of the holding member 52 is released. Since the valve seat member 40 is urged rearward by the spring 47, the urging force of the spring 47 immediately moves backward as soon as the engaged portion 40c is released.
[0044]
When the valve seat member 40 moves rearward, the second negative pressure valve seat 39 of the valve seat member 40 comes into contact with the valve seal portion 41a constituting the movable portion 41 of the valve mechanism 36, and the constant pressure chamber 23 and the variable pressure chamber 24 Block communication. At this time, since the plunger 37 is moving forward together with the input rod 28 and the valve seat member 40 pushes back the movable portion 41 of the valve mechanism 36, the valve 37 and the air valve seat 37a of the plunger 37 The valve seal portion 41b constituting the movable portion 41 of the mechanism 36 is rapidly separated, and the variable pressure chamber 24 communicates with the atmosphere. As a result, compared with the normal brake operation, the communication between the constant pressure chamber 23 and the variable pressure chamber 24 is rapidly cut off and the variable pressure chamber 24 is communicated with the atmosphere, and the power piston 22 is substantially connected to the reaction force member 54. The distance between the contact surface 22d and the valve seal portion 41b is increased, and the output in the jumping state can be made larger than that in the normal state.
[0045]
The normal brake characteristics and the emergency brake characteristics are shown in FIG. In FIG. 5, the jumping in the normal brake can only obtain the output of the magnitude F ₀₁ , but the jumping during the emergency braking increases to F ₀₁₁ , and the brake hydraulic pressure is sufficiently large with a small pedal effort. Can be generated.
[0046]
When the brake operation is completed and the brake pedal is returned, the plunger 37 moves rearward while the rear end surface 34b of the first flange portion 34 is in contact with the front surface of the key member 32. When the rear surface of the key member 32 comes into contact with the rear shell 12, the front surface of the key member 32 comes into contact with the front groove wall 40 f of the valve seat member 40, and the valve seat member 40 retracted integrally with the power piston 22. Regulates backward movement. At this time, since the power piston 22 is still retracted, the engaging portion 52b of the holding member 52 that retracts integrally with the power piston 22 is in contact with the rear shell 12 via the key member 32 and is stationary. The holding member 52 and the valve seat member 40 which are engaged with the engaged portion 40c of the seat member 40 and disengaged by the emergency brake operation are re-engaged to prepare for the next emergency brake operation. become.
[0047]
【The invention's effect】
According to the present invention, the retraction speed of the valve seat member is moderately moderated, and the negative pressure type multiple in which the contact sound due to the contact between the key member that retreats integrally with the valve seat member and the flange surface of the plunger is suppressed. It is possible to provide a force device.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a negative pressure booster according to an embodiment of the present invention.
FIG. 2 is a partial detail view of FIG. 1;
FIG. 3 is a perspective view before the valve seat member 40 is assembled.
FIG. 4 is a perspective view before the holding member 5 is assembled.
FIG. 5 is a front view of the retainer.
6 is a cross-sectional view taken along the line AA in FIG.
7 is a diagram showing the relationship between the force (input) applied to the input rod 28 and the propulsive force (output) of the output member 55. FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Negative pressure type booster 14 Housing 20 Movable wall 22 Power piston 23 Constant pressure chamber 24 Transformer chamber 28 Input rod 32 Key member 36 Valve mechanism 37 Plunger 37a Atmospheric valve seat 38 First negative pressure valve seat 39 Second negative pressure valve seat 40 Valve seat Member 40h Rear end face 41 Movable part 43 Fixed part 47 Spring (driving means, first biasing member)
48 Holding means 55 Output member 62 Spring (second biasing member)
62a Front end face

Claims (2)

A housing having at least one pressure chamber formed therein;
A movable wall installed inside the housing so as to be capable of moving forward and backward with respect to the housing and dividing the pressure chamber into a constant pressure chamber and a variable pressure chamber;
A power piston coupled to the movable wall;
An input rod installed inside the power piston so as to be movable forward and backward with respect to the power piston;
A plunger connected to the input rod and inserted into the power piston so as to be slidable in the front-rear direction with respect to the power piston, and an air valve seat is formed at the rear end;
An output member that outputs the propulsive force of the power piston accompanying the movement of the movable wall to the outside of the device;
A first negative pressure valve seat disposed on the power piston and facing rearward;
A second negative pressure valve seat formed on a valve seat member inserted through the plunger and capable of moving forward and backward with respect to the power piston and coaxial with the first negative pressure valve seat;
A valve mechanism having a movable part facing the first negative pressure valve seat and a fixed part airtightly fixed to the power piston;
The valve member has a first urging member that urges the valve seat member toward the movable portion, and the valve is moved by the urging force of the first urging member when an advance amount of the plunger with respect to the power piston is a predetermined value or more. Drive means for retracting the seat member toward the movable part;
Holding means for holding the valve seat member at a predetermined position in the power piston when an advance amount of the plunger relative to the power piston is smaller than a predetermined value;
In a negative pressure type booster comprising a key member inserted through the valve seat member and restricting the amount of movement of the plunger in the front-rear direction relative to the power piston through the radial hole of the power piston,
A retainer that is inserted into the plunger and is restricted from moving forward;
A second urging member compressed by the retainer and a rear flange portion of the plunger and applied with a predetermined set load;
When the advance amount of the plunger relative to the power piston becomes equal to or greater than a predetermined value , the urging force of the first urging member causes the valve seat member to retreat integrally with the key member, so that the key member A negative pressure booster that contacts the retainer before contacting the plunger, and further retracts a predetermined amount toward the movable portion against the urging force of the second urging member. In claim 1,
The second urging member is disposed on the same axis as the plunger, and has a front end surface portion that comes into contact with a rear end surface of the valve seat member.

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