JP3976104B2 - Drum brake wheel cylinder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a wheel cylinder which provides a brake which responses stably in ahead braking and responses at the same level in astern braking as that of the ahead braking in a duo-servo drum brake device. SOLUTION: A wheel cylinder comprises a primary piston 261 which presses a primary shoe 3 by hydraulic pressure of a pressure chamber 210, a secondary piston 262 which presses a secondary shoe 4 by hydraulic pressure of the pressure chamber 210, and a control piston 33 which stops hydraulic supply to the pressure chamber 210 so as to maintain anchor reactive force constant when the anchor reactive force reaches a predetermined magnification against hydraulic pressure from a master cylinder by the anchor reactive force of the secondary shoe 4 in ahead braking, wherein a diameter of the secondary piston 262 is formed to be shorter than that of the primary piston 261 so that pressured area for hydraulic pressure from the pressure chamber 210 is set to be smaller.

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention can be used in a duo-servo type drum brake device mounted on a vehicle or the like, thereby realizing a brake that has a stable effect at the time of forward braking and a level equivalent to that at the time of forward braking at the time of reverse braking. The present invention relates to a wheel cylinder of a drum brake device.
[0002]
[Prior art]
Conventionally, various types of drum brake devices have been used to brake the running of the vehicle, but these drum brake devices are arranged according to the arrangement of brake shoes pressed against the inner peripheral surface of a substantially cylindrical drum. It is classified as a reading trailing type, a two reading type, or a duo servo type.
[0003]
A duo-servo type drum brake device generally includes a pair of brake shoes of a primary shoe and a secondary shoe disposed in a cylindrical drum so as to face each other.
In the primary shoe, the front side in the forward rotation direction of the drum is used as an input unit, and the front side in the forward rotation direction of the drum is connected to the front side of the secondary shoe via an adjuster, for example. On the other hand, the front side of the secondary shoe is brought into contact with an anchor portion provided on the backing plate, and the anchor reaction force acting on the primary shoe and the secondary shoe is received by the anchor portion.
[0004]
As a result, when the primary shoe and the secondary shoe are expanded and pressed against the inner peripheral surface of the drum, the anchor reaction force acting on the primary shoe is input to the front side of the secondary shoe and the secondary shoe is moved into the drum. Since it acts to press against the peripheral surface, both the primary shoe and the secondary shoe operate as leading shoes, and a braking force with a very high gain can be obtained.
[0005]
[Problems to be solved by the invention]
The duo-servo type drum brake device described above can not only obtain extremely high braking force compared to the leading trailing type and two-leading type drum brake devices, but also can be easily miniaturized and can also incorporate a parking brake. It has many advantages such as being easy.
However, such a duo-servo drum brake device is sensitive to changes in the friction coefficient of the brake shoe lining, and therefore tends to be difficult to stabilize the braking force, and a device for stabilizing the braking force is required. .
[0006]
In addition, the duo-servo drum brake device can provide the same servo effect regardless of whether the drum rotates forward or backward, so that the same braking force can be obtained regardless of the drum rotation direction. As such, consideration is an important issue.
[0007]
The present invention has been made in view of the above circumstances. For example, when applied to a duo-servo type drum brake device, the brake has a stable effect at the time of forward braking, and realizes a brake whose effect level at the time of reverse braking is equal to that at the time of forward braking. It is an object of the present invention to provide a wheel cylinder of a drum brake device that can be used.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, a wheel cylinder of a drum brake device according to the present invention is disposed between one opposed end of a pair of primary shoes and a secondary shoe disposed opposite to each other in a drum inner space. In the wheel cylinder of the drum brake device that expands the cylinder body, the cylinder body has a first cylinder portion whose base end communicates with the pressure chamber and opens to one end of the cylinder body, and a base end communicates with the pressure chamber. the communication with the second cylinder portion which is open at the other end of the cylinder body, a control chamber which fluid pressure from the master cylinder is provided to communicate with the pressure chamber is supplied, the proximal end said control chamber Te with a control cylinder which is open at the other end of the cylinder body, the hydraulic pressure supplied to the pressure chamber is slidably retained in the first cylinder portion A primary piston that presses the primary shoe to the drum side, and a secondary piston that presses the secondary shoe to the drum side by the hydraulic pressure that is slidably held in the second cylinder portion and supplied to the pressure chamber. And a slidably held by the control cylinder portion, the base end portion receives the hydraulic pressure from the control chamber, and the tip end portion contacts the secondary shoe via the control lever, and the control lever during forward braking A control piston that is displaced into the control chamber when an anchor reaction force acting from the secondary shoe becomes greater than a certain value, and a displacement of the control piston into the control chamber that is installed in the control chamber. A valve mechanism for stopping the hydraulic pressure supply, and the secondary piston has an area for receiving the hydraulic pressure in the pressure chamber. Characterized by being smaller than the Imari piston.
[0009]
According to the above configuration, at the time of forward braking, the primary piston presses the primary shoe against the drum side by the hydraulic pressure supplied from the master cylinder to the pressure chamber through the control chamber, and at the same time, the secondary piston drums the secondary shoe. The brake shoes are expanded and pressed against the inner peripheral surface of the drum. At this time, since the anchor reaction force acting on the primary shoe is input to the inlet side of the secondary shoe and acts to press the secondary shoe against the drum inner peripheral surface, both the primary shoe and the secondary shoe are leading.・ It operates as a shoe and can obtain a braking force with a very high gain.
During this forward braking, the anchor reaction force of the secondary shoe acts on the control piston via the control lever, and the anchor reaction force acting on the control piston reaches a predetermined magnification and acts on the base end portion of the control piston. When the fluid pressure exceeds the hydraulic pressure, the control piston is displaced toward the control chamber, and the valve mechanism in the control chamber stops supplying the hydraulic pressure to the pressure chamber. The effectiveness of the braking force can be stabilized by maintaining the predetermined magnification.
[0010]
On the other hand, even during reverse braking, the primary and secondary pistons are actuated by the hydraulic pressure supplied from the master cylinder to the pressure chamber through the control chamber, and each brake shoe is expanded and pressed against the drum, generating braking force. To do. However, at the time of reverse braking, the anchor reaction force of the secondary shoe acts as an input of the primary shoe to generate a braking force with a high gain.
During this reverse braking, the anchor reaction force of the primary shoe is received by the anchor portion, the primary piston, etc. with which the primary shoe abuts, and control of the anchor reaction force by the control piston cannot be obtained. However, compared to the primary piston that is the shoe drive piston during forward braking, the secondary piston that is the shoe drive piston during reverse braking has a smaller area for receiving the hydraulic pressure in the pressure chamber, and the secondary piston is the secondary piston.・ Since the pressing drive force transmitted to the shoe itself is limited, the anchor reaction force input from the secondary shoe to the primary shoe can be kept small, which causes the primary shoe anchor reaction force to become excessive. Can be avoided. By setting the pressure receiving area in the pressure chamber of the secondary piston to be appropriately smaller than that of the primary piston, it is possible to realize a brake in which the effective level of the braking force is the same as that during forward braking during reverse braking.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, preferred embodiments of a wheel cylinder of a drum brake device according to the present invention will be described in detail with reference to the drawings.
FIG. 1 is a main part front view showing a schematic configuration of an embodiment of a duo-servo type drum brake device using a wheel cylinder according to the present invention, and FIG. 2 is an enlarged sectional view of the wheel cylinder shown in FIG.
[0012]
The drum brake device 1 includes a pair of brake shoes 3 and 4 of a primary shoe 3 and a secondary shoe 4 that are opposed to each other in a substantially cylindrical space in a drum (not shown), and the brake shoes 3 and 4. A wheel cylinder 2 that is disposed between one opposed end and expands each brake shoe 3, 4 and an output of the primary shoe 3 disposed between the other opposed ends of these brake shoes 3, 4. An adjuster 6 that also functions as a link that inputs to the secondary shoe 4, a backing plate 7 that supports these components, and anchor pins 9 and 10 that are fixedly installed at both ends of the cylinder body 21 of the wheel cylinder 2 ( 2).
[0013]
The brake shoes 3 and 4 are attached to the outer circumferences of the rims 3a and 4a, arcuate plate-like rims 3a and 4a extending along the inner peripheral surface of the drum, webs 3b and 4b protruding from the rims 3a and 4a toward the inner diameter side Linings 3c and 4c. Each brake shoe 3, 4 is attached to the backing plate 7 by a support member 12 penetrating the webs 3 b, 4 b so as to be able to advance and retreat toward the inner peripheral surface of the drum.
Further, the opposing ends of the brake shoes 3 and 4 on the backing plate 7 are urged by the return springs 14 and 15 toward each other (ie, away from the drum).
[0014]
On the backing plate 7, a parking strut 71 constituting a parking brake and a parking lever (not shown) are also incorporated. The parking lever is rotatably connected to the backing plate 7 by a parking lever pin (not shown), and the brake shoes 3 and 4 can be pressed against the drum also by rotating the parking lever.
[0015]
The adjuster 6 originally adjusts the distance between the ends of the brake shoes 3 and 4 in accordance with the progress of wear of the linings 3c and 4c of the brake shoes 3 and 4. That is, the distance between the ends of the brake shoes 3 and 4 is automatically adjusted by the operation of the adjuster lever whose tip is in contact with the adjusting gear 6a on the adjuster 6 by the biasing force of the adjuster spring 16. Yes.
The drum (not shown) is concentric with the backing plate 7 and rotates in the direction of arrow R in FIG. 1 when the vehicle moves forward.
[0016]
As shown in FIG. 2, the wheel cylinder 2 is a cylinder body 21 in which a primary piston 261, a secondary piston 262, a control piston 33, a valve mechanism 48, and the like are assembled. The cylinder body 21 has a first cylinder portion 211 whose base end communicates with the pressure chamber 210 and opens to one end side of the cylinder body 21 , and a base end which communicates with the pressure chamber 210 and opens to the other end side of the cylinder body 21. A control chamber 214 that communicates with the pressure chamber 210 via the communication passage 215 and is supplied with hydraulic pressure from the master cylinder, and a base end that communicates with the control chamber 214 and has a cylinder body 21. And a control cylinder portion 216 that opens to the other end side. The cylinder body 21 is connected to the backing plate 7 by a support shaft 28, and anchor pins 9 and 10 are fitted to both ends thereof. The first cylinder part 211 and the second cylinder part 212 are formed on substantially the same straight line.
[0017]
The primary piston 261 is slidably held by the first cylinder portion 211 of the cylinder body 21 and is urged toward the primary shoe 3 by a spring 29 interposed between the secondary piston 262 in the pressure chamber 210. The primary shoe 3 is brought into contact with the primary shoe 3 via the primary rod 30, and the primary shoe 3 is pressed and driven by the hydraulic pressure supplied to the pressure chamber 210 during braking.
The anchor reaction force of the primary shoe 3 during reverse braking is distributed to the primary piston 261 that is in contact with the primary shoe 3 via the primary rod 30 and the anchor pin 9.
The primary rod 30 is equipped with a boot 61 that prevents foreign matter from entering the primary piston 261 side. Further, the primary piston 261 and the first cylinder portion 211 are sealed with a seal member 28a fitted to the outer periphery of the primary piston 261.
[0018]
The secondary piston 262 is slidably held in the second cylinder portion 212 of the cylinder body 21 and is urged toward the secondary shoe 4 by a spring 29 interposed between the primary piston 261 in the pressure chamber 210. The secondary shoe 4 is in contact with the secondary shoe 4 via the secondary rod 31 and the control lever 35 , and the secondary shoe 4 is pressed and driven by the hydraulic pressure supplied to the pressure chamber 210 during braking. The anchor reaction force of the secondary shoe 4 during forward braking is applied to the control lever 35 via the anchor pin 10 and the secondary rod 31 by the control lever 35 provided between the wheel cylinder 2 and the secondary shoe 4. The secondary piston 262 is in contact with the control piston 33 and the control piston 33 is in contact with the control lever 35 via the control rod 32. The secondary rod 31 is equipped with a boot 62 that prevents foreign matter from entering the secondary piston 262 side. Further, the space between the secondary piston 262 and the second cylinder portion 212 is sealed by a seal member 28b fitted to the outer periphery of the secondary piston 262.
[0019]
The control piston 33 is slidably held by the control cylinder portion 216 of the cylinder body 21 and receives the hydraulic pressure from the master cylinder at the base end portion located in the control cylinder portion 216 during braking. A communication hole 33a is formed through the peripheral wall portion of the control piston 33 to allow communication between the communication passage 215 formed in the cylinder body 21 and the inside of the piston.
Further, a cylindrical guide 38 and a plug 39 that are fitted and locked to the front end side of the control cylinder portion 216 are fitted to the front end side of the control piston 33. A seal member 28 c is provided between the guide 38 and the plug 39 so as to be in close contact with the inner peripheral surface of the control cylinder portion 216 and the outer peripheral surface of the control piston 33.
[0020]
The plug 39 has a structure having a flange 39 a with which the outer peripheral portion of the tip of the control piston 33 abuts, and is prevented from being removed by a ring-shaped clip 40 fitted to the inner peripheral surface of the control cylinder portion 216.
Therefore, the position where the control piston 33 abuts on the flange 39a becomes the limit of movement of the control cylinder 216 toward the tip side.
[0021]
Further, the control piston 33 is urged toward the tip end side (open end side) of the control cylinder portion 216 by a spring 41 interposed between the step portion in the control cylinder portion 216. One end of the spring 41 is in contact with a stepped portion in the control cylinder portion 216 via a spring seat (spring seat) 41a, and the other end is in contact with an end surface of the control piston 33 via a spring seat 41b.
[0022]
Further, the tip of the control piston 33 is brought into contact with the control lever 35 via the control rod 32 inserted through the plug 39, and a part of the anchor reaction force of the secondary shoe 4 is transmitted via the control lever 35 during forward braking. Receive. The control rod 32 is equipped with a boot 63 that prevents foreign matter from entering the control piston 33 side.
[0023]
That is, the control piston 33 configured as described above receives the hydraulic pressure in the control chamber 214 (that is, the hydraulic pressure from the master cylinder) on the base end side, and receives the anchor reaction force from the secondary shoe 4 on the distal end side. When the anchor reaction force reaches a predetermined magnification and the urging force due to the anchor reaction force exceeds the urging force due to the hydraulic pressure, the anchor reaction force is displaced into the control chamber 214 and moved to the pressure chamber 210 by the valve mechanism 48 provided in the control chamber 214. The communication path 215 is closed.
[0024]
A cup seal 43 that comes into contact with the inner peripheral surface of the control cylinder part 216 is provided on the outer periphery of the control piston 33. The cup seal 43 is set with the opening directed to the control chamber 214 side.
[0025]
The control lever 35 is connected to the anchor engaging portion 35 a that is rotatably contacted with the anchor pin 10 on the cylinder body 21, the first rod engaging portion 35 b that is rotatably contacted to the secondary rod 31, and the control rod 32. A second rod engaging portion 35c that contacts the second shoe 4 is provided on one side, and a shoe engaging portion 35d that contacts the end of the secondary shoe 4 is provided at the intermediate portion on the other end side. Thus, during braking, the pressing force transmitted from the secondary piston 262 via the secondary rod 31 is transmitted to the secondary shoe 4. Further, the anchor reaction force of the secondary shoe 4 during forward braking is distributed to the anchor pin 10, the secondary rod 31, and the control rod 32 at a predetermined ratio.
[0026]
The valve mechanism 48 includes a substantially axial valve seat 49 slidably fitted in the hollow portion of the control piston 33, and the valve seat 49 follows the displacement of the control piston 33 toward the control chamber 214. And a valve body 50 that is seated on the valve seat 49 when the valve is displaced by a predetermined amount.
In the valve seat 49, a communication hole 49 a that communicates the communication hole 33 a of the control piston 33 with the control chamber 214 is formed along the central axis, and the control piston 33 is interposed between the valve seat 49 and the control piston 33. A damper spring 55 is provided to relieve an impact when the control piston 33 hits the valve seat 49 when displaced to the control chamber 214 side. Further, a seal ring 56 that seals between the valve piston 49 and the control piston 33 is provided on the outer periphery of the valve seat 49.
[0027]
The valve body 50 is accommodated and held so as to be movable in the axial direction of the control cylinder portion 216 by a bowl-shaped valve case 52 held in the control chamber 214 via the spring seat 41b. It is urged toward the valve seat 49 by a valve spring 53 mounted on the inner bottom side. Further, the valve body 50 is formed of an appropriate elastic material such as a rubber material, and a spherical portion 50a for closing the communication hole 49a of the valve seat 49 protrudes from the tip. Further, in order to prevent excessive deformation of the valve body 50 when the communication hole 49a is closed, a valve plate 50b that is in contact with the end surface of the valve seat 49 is mounted on the distal end surface of the valve body 50. Further, the valve plate 50b is engaged with the valve case 52 so that the valve body 50 is not seated on the valve seat 49 until the displacement amount of the control piston 33 toward the control chamber 214 becomes a predetermined value or more. The movement of 50 toward the valve seat 49 is limited.
[0028]
As is apparent from the configuration of the valve mechanism 48 described above, the communication path 215 is connected to the control chamber 214 via the communication hole 33a formed through the control piston 33 and the communication hole 49a formed through the valve seat 49. When the communication hole 49a of the valve seat 49 is closed by the valve body 50 due to the displacement of the control piston 33, the pressure chamber 210 and the control chamber 214 are disconnected from each other. The hydraulic pressure supply to the pressure chamber 210 is stopped.
[0029]
In the wheel cylinder 2 described above, during forward braking in which the drum rotates in the direction of arrow R in FIG. 1, the primary piston 261 expands the primary shoe 3 with the hydraulic pressure supplied from the master cylinder to the pressure chamber 210 via the control chamber 214. Open the primary shoe 3 against the drum inner surface. At this time, the anchor reaction force acting on the primary shoe 3 is input to the front side of the secondary shoe 4 so as to press the secondary shoe 4 against the drum inner peripheral surface. Both of the shoes 4 operate as leading shoes, and a braking force with a very high gain can be obtained.
[0030]
Then, the anchor reaction force of the secondary shoe 4 acts on the front end portion of the control piston 33 via the control lever 35 and the control rod 32, and the anchor reaction force reaches a predetermined magnification and is applied to the front end portion of the control piston 33. When the acting anchor reaction force becomes larger than the hydraulic pressure acting on the base end portion of the control piston 33, the control piston 33 is displaced toward the control chamber 214, and the valve body 50 is seated on the valve seat 49. In this state, the communication path 215 communicating between the control chamber 214 and the pressure chamber 210 is closed, and the supply of hydraulic pressure to the pressure chamber 210 is stopped, so that the urging force of the primary piston 261 is reduced. Held constant. Accordingly, it is possible to stabilize the effectiveness of the braking force by maintaining the anchor reaction force at a predetermined magnification with respect to the hydraulic pressure from the master cylinder.
[0031]
From the state where the valve body 50 is seated on the valve seat 49, the brake pedal is further pressed and the hydraulic pressure is further supplied from the master cylinder, whereby the hydraulic pressure in the control chamber 214 is increased, and the control piston by the hydraulic pressure is increased. When the urging force to 33 becomes larger than the anchor reaction force from the secondary shoe 4, the control piston 33 is pushed back to the secondary shoe 4 side, and the valve seat 49 is detached from the valve body 50. The communication state is established again, and a larger braking force is generated by supplying the hydraulic pressure in the pressure chamber 210.
[0032]
Further, at the time of reverse braking, the secondary piston 262 that operates with the hydraulic pressure supplied to the pressure chamber 210 presses the secondary shoe 4 against the drum inner peripheral surface via the secondary rod 31 and the control lever 35. At this time, the anchor reaction force acting on the secondary shoe 4 is input to the inlet side of the primary shoe 3 to press the primary shoe 3 against the drum inner peripheral surface. Both the primary shoe 3 and the primary shoe 3 operate as leading shoes, and a very high gain braking force can be obtained.
During the reverse braking, the anchor reaction force of the primary shoe 3 is received by the anchor portion 9 and the primary piston 261 with which the primary shoe 3 abuts, and the control of the anchor reaction force by the control piston 33 cannot be obtained.
[0033]
However, compared to the primary piston 261 that is a shoe drive piston at the time of forward braking, the secondary piston 262 that is a shoe drive piston at the time of reverse brake is set to have a smaller area for receiving the hydraulic pressure in the pressure chamber 210, and the secondary piston 262 Since the pressing drive force itself that the piston 262 transmits to the secondary shoe 4 is limited, the anchor reaction force input from the secondary shoe to the primary shoe can be suppressed to a small value. An excessive force can be avoided.
[0034]
By setting the pressure receiving area of the secondary piston 262 in the pressure chamber 210 to be appropriately small, it is possible to realize a brake in which the level of effectiveness is the same as that during forward braking during reverse braking.
Therefore, when the wheel cylinder 2 is used in the duo-servo type drum brake device 1 as in this embodiment, it can have the same braking force regardless of the rotation direction of the drum.
In addition, it cannot be overemphasized that the design etc. of the shape of each component in the above one embodiment can be changed suitably in the range which does not deviate from the meaning of this invention.
[0035]
【The invention's effect】
According to the wheel cylinder of the drum brake device of the present invention, during forward braking, the anchor reaction force of the secondary shoe acts on the control piston via the control lever, and the anchor reaction force acting on the control piston reaches a predetermined magnification. When the hydraulic pressure acting on the base end of the control piston becomes larger, the control piston is displaced to the control chamber side, and the valve mechanism in the control chamber stops supplying the hydraulic pressure to the pressure chamber. The effectiveness of the braking force can be stabilized by maintaining a predetermined magnification with respect to the hydraulic pressure from the master cylinder.
On the other hand, at the time of reverse braking, the anchor reaction force of the primary shoe is received by the anchor portion, the primary piston, etc. with which the primary shoe abuts, and the control of the anchor reaction force by the control piston cannot be obtained. However, compared to the primary piston that is the shoe drive piston during forward braking, the secondary piston that is the shoe drive piston during reverse braking has a smaller area for receiving the hydraulic pressure in the pressure chamber, and the secondary piston is the secondary piston.・ Since the pressing drive force transmitted to the shoe itself is limited, the anchor reaction force input from the secondary shoe to the primary shoe can be kept small, which causes the primary shoe anchor reaction force to become excessive. Can be avoided. By setting the pressure receiving area in the pressure chamber of the secondary piston to be appropriately small, the effect of the braking force can be set to the same level as that during forward braking even during reverse braking.
Therefore, when the wheel cylinder described above is used in, for example, a duo-servo type drum brake device, it is possible to realize a brake having a stable effect at the time of forward braking and a brake effect level equivalent to that at the time of forward braking at the time of reverse braking.
[Brief description of the drawings]
FIG. 1 is a main part front view showing a schematic configuration of an embodiment of a duo-servo type drum brake device using a wheel cylinder according to the present invention.
FIG. 2 is an enlarged cross-sectional view of the wheel cylinder shown in FIG.
[Explanation of symbols]
1 Drum brake device 2 Wheel cylinder 3 Primary shoe (brake shoe)
4 Secondary shoe (brake shoe)
6 Adjuster 7 Backing plate 9, 10 Anchor pin 21 Cylinder body 30 Primary rod 31 Secondary rod 32 Control rod 33 Control piston 35 Control lever 48 Valve mechanism 49 Valve seat 50 Valve body 71 Parking strut 211 First cylinder part 212 Second cylinder part 210 Pressure chamber 214 Control chamber 215 Communication path 261 Primary piston 262 Secondary piston

Claims (1)

In a wheel cylinder of a drum brake device that is disposed between one opposed end of a pair of primary shoes and a secondary shoe that are disposed opposite to each other in the drum inner space, and that expands the brake shoes, the cylinder body has a base end A first cylinder portion that communicates with the pressure chamber and opens to one end side of the cylinder body, a second cylinder portion whose proximal end communicates with the pressure chamber and opens to the other end side of the cylinder body, and the pressure A control chamber provided in communication with the chamber and supplied with hydraulic pressure from a master cylinder; and a control cylinder portion having a proximal end communicating with the control chamber and opening on the other end side of the cylinder body. A primary piston that presses the primary shoe toward the drum side by a hydraulic pressure that is slidably held in one cylinder portion and is supplied to the pressure chamber; A secondary piston that presses the secondary shoe toward the drum side by hydraulic pressure that is slidably held in the cylinder portion and supplied to the pressure chamber, and a base end portion that is slidably held in the control cylinder portion When the hydraulic pressure from the control chamber is received and the tip portion comes into contact with the secondary shoe via the control lever, and when the anchor reaction force acting from the secondary shoe via the control lever becomes greater than a certain level during forward braking A control piston that is displaced in the control chamber; and a valve mechanism that is provided in the control chamber and stops the supply of hydraulic pressure to the pressure chamber by displacement of the control piston into the control chamber. An area for receiving the hydraulic pressure in the pressure chamber is set smaller than that of the primary piston. Rushirinda.

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