GB2073346A - Hydraulic brake boosters - Google Patents

Abstract

A hydraulic brake booster is supplied from a pump 12 and incorporates an accumulator 300 connected to the booster through valve 330. The valve 330 is opened by movement of the power piston 112, and accumulator pressure is then transmitted to passage 174 and, under control of control valve 178, to the booster chamber 114. <IMAGE>

Description

SPECIFICATION Power booster in a brake system for an automotive vehicle The present invention relates generally to a power booster in the brake system for an automotive vehicle. More specific Ily, the invention relates to a working fluid supply system for a hydraulically operative power booster in a vehicle brake system, which system can supply the working fluid to the booster even when the fluid source is damaged.

The hydraulic power booster to be provided in the vehicie. brake system has been known that can build up a greater boost force to be transmitted to a brake system master cylinder in relation to other types of brake power boosters. Also, the hydraulic power booster can be sized smaller than the other kinds of power booster such as vacuum type power booster. Generally, the hydraulic power booster comprises a booster housing communicating with a fluid source, a power piston slidably disposed within the booster housing and a valve member connected with a brake pedal and slidable within the power piston.

The boost pressure is built up in the booster housing by the valve member motion corresponding to the magnitude of the depression of the brake pedal. The fluid source generally comprises a fluid pump driven by the engine.

In such kind of booster, it is essential to be supplied the working fluid with a sufficient fluid pressure from the fluid source. Therefore, if the fluid pump forcing the working fluid to the booster is damaged or hydraulic circuit connecting between the fluid source and the booster is interrupted, the booster may not work for building up the boost force.

If the booster cannot work, the brake force to be applied to the brake pedal become unnecessarily increased so that the brake system possibly works insufficiently. Therefore, it has been required for the hydraulic power booster for supplying the working fluid with a sufficient pressure even when the fluid source or the hydraulic circuit is damged.

SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a hydraulic power booster which has a means for accumulating the fluid pressure while the working fluid is supplied to the booster in normal condition and for supplying the accumulated pressurized fluid to the booster for building up the boost pressure in the booster even when the fluid source is damaged or disconnected from the booster.

To accomplish the above-mentioned and other objects of the invention, there is provided a hydraulic power booster incorporating therewith an accumulating means for accumulating a fluid pressure therein. The accumulating means is communicated with the fluid source and is supplied therefrom the working fluid in order to accumulate the fluid pressure therein. The accumulating means is also connected with the booster via a one-way check valve for normally interrupting fluid flow from the accumulating means to the booster. The one-way check valve member is openable in response to an application of the brake pedal for applying the brake to establish communication between the accumulating means and the booster for permitting the fluid flow from the accumulating means to the booster.

Another one-way check valve means is interpositioned between the fluid source and the booster for interrupting the fluid flow from the booster to the fluid source through the fluid supply passage.

According to one embodiment of the present invention, there is provided a hydraulic power booster in a brake system for an automotive vehicle comprising a booster housing communicating with a working fluid source and introducing therefrom a working fluid, which working fluid is recirculated through a hydraulic circuit via the booster housing, a power piston slidably disposed within the booster housing and movable along the longitudinal axis of the housing, the power piston being connected with a brake master cylinder which distributes pressurized fluid to each of individual wheel cylinders, the power piston defining a pressure chamber within the booster housing where a boost pressure is built up, a valve member slidably disposed within the power piston and cooperative with the power piston for connecting and disconnecting the pressure chamber with respect to the fluid source corresponding to application and release of a brake pedal, a pressure accumulating means communicating with the fluid source to be supplied therefrom the working fluid and for accumulating the fluid pressure therein, the pressure accumulating means being communicating with the booster housing for supplying the pressurized working fluid to the booster housing, and a check valve means interpositioned between said booster housing and the pressure accumulating means for normally interrupting fluid flow from the accumulating means to the booster housing and for opening in response to motion of the power piston to permit fluid flow from the pressure accumulating means to the booster housing.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the present invention, which, however, should not be taken as limitative to the invention but for elucidation and explanation only.

In the drawings: Figure 1 its an illustration of a preferred embodiment of a hydraulic booster including a hydraulic circuit thereof, in which the booster is sectioned generally along the longitudinal axis for explanation; Figure 1A is an enlarged partial section of the booster of Fig. 1; Figure 2 is an enlarged transversary section of the hydraulic booster of Fig. 1 showing the greater detail of the booster; and Figure 3 is a similar view to Fig. 2 but showing another embodiment of the present invention DESCRIPTION OF THE PREFERRED EMBODI MENTS Referring now to the drawings, particularly to Fig. 1, there is illustrated a general construction of a hydraulic system in which a preferred embodiment of a hydraulic booster according to the present invention is interpositioned.Working fluid is generally stored in a fluid reservoir 10. A fluid pump 1 2 communicates with the fluid reservoir 10 through a fluid suction passage 1 4. The fluid pump 1 2 is mechanically connected to an engine (not shown) such as an internal combustion engine in the per se known manner to be driven thereby. The fluid pump 1 2 pressurizes the working fluid and feeds it to an inlet port 1 8 of a hydraulically operative booster 20. The hydraulic booster 20 cooperates with a brake pedal 21 and with a braking system master cylinder (not shown).

In a fluid passage 24 connecting the fluid pump 1 2 to the hydraulic booster inlet port 18, there is provided a check valve 26.

Through the check valve 26, the working fluid is also fed to an inlet port 302 of an accumulator 300 for accumulating the fluid pressure therein, through a fluid passage 28 via a check valve 30 and an orifice 32. Further, the inlet port 302 of the accumulator.300 is connected with a port 304 through a passage 306 branched from the fluid passage 28.

The working fluid applied to the hydraulic booster 20 is exhausted from outlet and exhaust ports 48 and 50 which communicate with drain passages 54 and 52 respectively.

In the drain passage 54, there is interposed an operation valve 55 of a hydraulically operative power steering system 56. The power steering system generally comprises the operation valve 55 having a pair of working sections (not shown) operative by pressurized fluid. When a driver operates the operation valve 55 via steering wheel 62, the working fluid is applied to either of the working sections to operate the vehicle steering mechanism.

The brake master cylinder connected with the booster 20 has its own independent hydraulic system different from the hydraulic system illustrated herewith. By applying brake pedal force and providing boosting force from the hydraulic booster, the master cylinder operates to apply pressurized fluid to each Qf the vehicle wheel braking discs.

As stated hereinabove, the hydraulic power booster 20 is connected to the hydraulic fluid pump 1 2 and to the steering system operation valve 55. The hydraulic booster 20 includes a generally cylindrical-shaped housing 102 having an inlet port 18, outlet port 48 and drain port 50. The inlet port 1 8 communicates with the outlet of the fluid pump 1 2 through the fluid passage 24 and the check valve 26 and therefore, the pressurized fluid is applied to the hydraulic booster 20 therethrough. Upon delivering the pressurized fluid to the hydraulic booster 20, the relief valve 66 regulates the fluid pressure to a predetermined level.

The hydraulic booster fluid is drained through the housing drain port 50 during the release of the brake pedal 21. The outlet'port 48 drains the excess fluid therethrough when the boost force applied to the brake master cylinder exceeds a predetermined amount.

When the brake pedal 21 is released, the pressure fluid flows from the inlet port 1 8 to the outlet port 48. The pressure fluid ex-hausted from the outlet port 48 is fed to the steering system operation valve 55.

As shown in Fig. 1, the booster housing 102 includes a stepped bore having two portions 108 and 110. The bore 108 slidably receives a cylindrical-shaped power piston 11 2 to divide the bore 108 into a pressure chamber 114 and an exhaust chamber 116.

A plug 11 8 with an annular sealing member 1 20 sealingly closes the end of the bore 108 adjacent to the exhaust chamber 11 6. The plug 11 8 is provided with a central opening 1 22 having an internal annular groove 124 which receives an annular sealing member 1 26 through which a force transmitting rod 1 28 slidingly projects to connect the piston 11 2 to a tandem brake master cylinder.

The end of the power piston 11 2 adjacent the exhaust chamber 11 6 is formed with a blind bore 1 27. An annular snap ring 129 retains the power transmitting rod 1 28 in position with the power piston 11 2. Preferably, however not shown in the drawings, the power piston 11 2 is further formed with 6 longitudinal channel on the outer periphery thereof and a bolt projects inwardly into the channel from the booster housing 102 to prevent the piston from rotating.

A piston return spring 1 44 is disposed in the housing exhaust chamber 11 6 to urge the piston 11 2 toward the pressure chamber 114 to the right in the drawings against a housing internal annular surface 146. The piston 11 2 is formed with longitudinal channels 148 and 1 60 on the outer periphery thereof. The channel 1 48 is positioned adjacent to a port 304 to communicate therewith under all conditions of operation. On the other hand, the channel 1 60 communicates with the exhaust chamber 11 6.

As shown in Figs. 1A, the piston 112 is formed with annular recesses 162, 1 63 and 1 64 respectively located between lands 166, 167, 1 68 and 1 69 on the internal surface thereof. Each recess 162, 163 and 164 opens toward a longitudinal bore 1 72 formed in the piston 11 2. The piston 11 2 includes radial passages 1 74 and 1 76 providing communication between the bore 1 72 and the longitudinal channels 1 48 and 160, respectively. The control valve means 1 78 is formed with lands 188, 190 and 192 defining annular recesses 184 and 186.The control valve means 1 78 is further provided with a stepped bore 1 94 communicating with the recess 1 86 through a passage 1 82 and with the pressure chamber 114 through a passage 228. In the released position of the control valve, as shown in Fig. 1, the first spool valve 1 80 formed between the lands 1 66 and 1 90 is open to permit communication between the inlet port 1 8 and the outlet port 48 therethrough.

The shoulder 208 of the land 1 90 acts as a second spool valve 210 together with the shoulder 212 of the land 167. The second spool valve 210 controls communication between the inlet port 1 8 and the pressure chamber 114 via the annular recess 186. The shoulder 214 of the land 192 acts as a third spool valve 218 together with the shoulder 216 of the land 167.Through the third spool valve 218, the recess 163 establishes communication between the pressure chamber 114 and the exhaust chamber 11 6 and, in turn, the pressure chamber 11 4 and the fluid reservoir 10 via the passage 1 76 radially formed in the piston 11 2 and the channel 1 60. In the released position shown in Fig. 1, the control valve means annular recess 1 86 and the piston internal land opposite it are offset to establish communication between the control valve means stepped bore 1 94 and the piston annular recess 1 63. In this position, fluid communication is establish from the pressure chamber 114, through the control valve passage 228 the stepped bore 1 94 to the left end of control valve means 1 78. In this position the control valve annular recess 1 86 is closed to pressurized fluid from the inlet port 18.Since pressurized fluid is preeluded from flowing into the pressure chamber 114, and the pressurized fluid therein is permitted to flow through the control valve, in this released position of Fig. 1, the control valve is permitted to shift to the right, releasing the force applied to the tandem master cylinder.

The power piston 11 2 includes an annular connecting ring 220 at the internal surface of the end adjacent to the pressure chamber 114. The ring 220 engages a flange portion 222 of an outer cylindrical member 224 of a plunger member 248 slidably received in the stepped bore 110 of the housing 1 02.

A helical spring 232 is disposed within the portion 1 98 of the valve means bore 1 94 to urge the control valve toward the outer cylindrical member 224. One end of the spring 232 is seated onto the resilient member 236 positioned in the bottom of the bore 1 94. The resilient member 236 is formed from resilient material such as urethane rubber and is formed with an annular projective portion 238 on the surface adjacent to the valve member 1 78. The thickness of the resilient member 236 corresponds to a distance between the front end of the spool valve 1 78 and the bottom of the piston bore 1 72 at the position where the shoulder 204 of the land 1 90 contacts the shoulder 206 of the land 1 66 to close the first valve 1 80. Thus, immediately after the first valve 1 80 closes, the front end of the spool valve 1 78 engages the top of the resilient member 236 so that the force applied to the brake pedal 21 is transmitted to the tandem master cylinder through the resilient member 236.

On the inner surface of the bore at the position 110, an annular recess 240 is formed to receive therein an annular sealing member 242. The inner surface of the sealing member 242 contacts the outer surface of the plunger member 248 for fluid sealing.

The booster housing 102 is fitted therewith an accumulator housing 308 having the inlet port 302, as shown in Fig. 1. The accumulator housing 308 is formed with a bore 310 therein. An open end of the bore 310 is closed by an end fitting 312 with annular sealers 314 and 316, likewise to the bore in the booster housing. A cup-shaped piston 318 is slidably disposed within the bore 310.

The piston 318 is slidable along the longitudinal axis of the bore 310 but accomplish liquid proof between the outer periphery thereof and the inner periphery of the bore. The piston 318 thus defines a chamber 320 in which is filled a gas. The piston 318 is formed with a cut-out 322 on the outer circumferential edge of the bottom thereof in order to define a minimum annular space of a fluid chamber 324 to receive the working fluid introduced through the inlet port 302. For introducing the working fluid into the fluid chamber 324, the inlet port 302 is located at a position corresponding to the fluid chamber 324.

The piston 31 8 is movable toward the chamber 320 corresponding to the fluid pressure built up within the fluid chamber 324.

By the piston motion responsive to the fluid pressure, the space of the chamber 320 is reduced with increasing of the gas pressure therein. Since the piston 31 8 is positioned to balance the gas pressure in the chamber 320 and the fluid pressure in the fluid chamber 324, the gas pressure accumulated in the chamber 320 corresponds to the fluid pressure applied in the fluid chamber 324.

As shown in Fig. 1, the port 302 is communicated with the port 304 through the passages 28 and 306. A check valve 330 is provided within the port 304, which check valve 330 is in a closed position during the release of the brake pedal. As shown in detail in Fig. 2, the check valve 330 comprises a valve housing 332 received within a through opening 334 formed in the booster housing 102 and communicating with the longitudinal channel 148. The valve housing 332 is formed with a thread on the outer periphery thereof, which thread is engaged with the thread formed on the inner periphery of the opening 334. The valve housing 332 is formed with a bore 336 therein, which communicates through a passage 338 with longitudinal groove 340 formed on the outer periphery thereof and defining a fluid passage 342 with the internal periphery of the opening 334.The fluid passage 342 serves for communication between the passage 338 and the longitudinal channel 148.

The valve housing 332 is further formed with a through opening 344 therein. A movable stem 346 is slidably placed within the opening 344. The movable stem 346 extends outwardly and downwardly from the opening 344 and contacts with ball members 348 and 350. The ball member 348 is normally placed within a groove 352 formed on the bottom of the channel 1 48 and having tapered sections at longitudinal both ends thereof. On the other hand, the ball member 350 serves as valve member and is normally seated onto a valve seat 354 with a spring 356. The opposite end of the spring 356 is seated on the inner end of a port member 358 engaged with the valve housing 332 with thread engagement.

As shown in Fig. 2, the check valves 26 and 30, and the relief valve 66 are also provided within the booster housing 1 02. A port 260 is formed in the booster housing 102 for communication between the fluid pump 1 2 and the'inlet port 18 via the fluid passage 24. In the fluid passage 24 is formed a valve seat 262 on which a ball-shaped valve member 264 is seated in normal state by urging force applied by a spring 266. The check valve 26 comprising the valve member 264 is so located that it can connect and disconnect the inlet port 1 8 with the fluid passage 24. The fluid passage 24 is connected with the fluid passage 28 via the check valve 30 and the orifice 32. The check valve 30 comprises a ball-shaped valve member 268 ancf a valve seat 270.The valve member 268 is normally urged toward the valve seat 270 by a spring 272.

The fluid passage 24 formed in the booster housing 102 is branched at a portion 274 upstream of the check valve 26. The relief valve 66 is provided within a branch passage 274 for communication between the fluid passage 24 and the outlet port 48. The outlet port 48 is formed with an enlarged diameter section 276 for maintaining the outlet port 48 connecting with the groove 1 84. The relief valve 66 comprises a valve member 278 having a stem portion 280 placed within the branch passage 274 for normally disconnecting the branch passage 274 from the outlet port 48. The valve member 278 is normally urged against a valve seat 282 by a spring 284 disposed within a bore 286 which is closed by a closure 288 at the outer end thereof.

In operation with the brake pedal 21 released, the relationship between the power piston 11 2 and the valve means 1 78 is as illustrated in Fig. 1. At this position, the working fluid is supplied through the fluid pump 12, the port 260, the fluid passage 24 and the check valve 26. The working fluid thus introduced into the booster 20 flows through the longitudinal recess 149, annular recess 162, the annular recess 1 84 and 6utlet port 48. The working fluid flows from the outlet port 48 to the operation valve 55 of the steering system.

At the same time, the working fluid pressurized through the fluid pump 1 2 is supplied to the accumulator 300 through the check valve 30 and orifice 32 and via fluid passage 28.

The working fluid introduced in the fluid chamber 324 through the port 302 urges the valve member 318 toward the chamber 320.

Thus, the gas pressure in the chamber 320 is increased until it balance with the fluid pressure to accumulate the pressure. The pressurized fluid in the fluid chamber 324 is held therein by the check valve 30 and the check valve 330.

On the other hand, the pressure chamber 114 is communicated with the exhaust chamber 11 6 through the passage 228, the bore 194, the radial passage 182, the annular recess 163, the radial passage 1 76 and the longitudinal channel 1 60. Therefore, no hydraulic pressure is built up in the pressure chamber 114 and the power piston 11 2 is thus urged toward the pressure chamber 114' by the spring pressure given by the return spring 144.

When the brake pedal 21 is applied, the pressure applied to the brake pedal 21 is transmitted to the valve means 1 78 through the plunger member 248. Thereby, the valve means 1 78 is moved toward the exhaust chamber 11 6 corresponding to the brake pressure applied to the brake pedal 21. During the valve member motion toward the exhaust chamber 11 6 the first and third spool valve 1 80 and 218 are closed at first. Thereafter the second spool valve 210 is opened.

By closing of the first spool valve 180, the hydraulic pressure is accumulated upstream thereof. Thus pressurized working fluid is then introduced into the pressure chamber 114 through the annular recess 186, the radial passage 182, the bore 1 94 and the passage 228. The boost pressure in the pressure chamber 114 is thus built up and forces the power piston 11 2 toward the exhaust chamber 116. The boost pressure built up in the pressure chamber 114 is transmitted to the master cylinder via the force transmitting rod 1 28 to operate the vehicle brake system. On the other hand, the boost pressure built up in the pressure chamber 11 4 is also transmitted to the brake pedal 21 via the plunger member 248 to give the brake pedal a reaction force.

When the boost pressure built up in the pressure chamber 114 exceeds a set pressure of the relief valve 66 defined by the set spring 284, the valve member 278 is forced toward the spring 284 against the spring force. Thus, the passage 274 is communicated with the drain port 48. The working fluid flows through the passage 274 and the drain passage 48 is supplied to the operation valve means 55 of the steering system 56 to provide a sufficient hydraulic pressure therefor.

This will aid for steering operation and reducing of the hydraulic pressure supplied via the first spool valve 1 80 and the drain port 48 may not influence to the steering operation.

At this time, the working fluid supplied to the booster 20 flows through the check valve 30, orifice 32 and port 302 into the fluid chamber 324. Thereby, the accumulator 300 accumulates a fluid pressure therein. Since the working fluid flowing into the pressure chamber 324 of the accumulator is limited by the orifice, the fluid pressure of the working fluid to be supplied to the booster 20 is not reduced so as to influence the working of the booster 20.

By motion of the power piston 11 2, the ball member 348 releases from the groove and pushed upwardly with pushing the movable stem 346 upwardly. By this, the ball member 350 disconnecting the passage 338 from the port 304 is moved upwardly. Therefore, pressurized fluid in the pressure chamber 324 of the accumulator 300 may be permitted to flow into the booster. The working fluid supplied from the accumulator 300 may flow through the port 304, the check valve 330 and the longitudinal recess 148, the radial passage 174, the annular recess 1 64 and a cut-out recess 165 formed at the end of the valve member 1 78 adjacent to the pressure chamber 114, longitudinal groove 194, the passage 228 into the pressure chamber 11 4.

If the fluid pump 1 2 or the fluid passage connecting the fluid source with the booster is damaged and therefore the working fluid is not supplied to the booster from the fluid source consisted of the fluid pump 1 2 and the fluid reservoir 10, the boost pressure may not be built up in the pressure chamber 114 with the working fluid from the fluid source. Therefore, until the valve member 1 78 is abut against the resilient member 236, the power piston 11 2 may not be moved and thus the brake force cannot be transmitted to the master cylinder dispite of application of brake force to the brake pedal 21.As mentioned above, the check valve 330 is opened to permit the working fluid pressurized in the pressure chamber 324 of the accumulator 300 to flow into the booster 20 through the port 304. Though the fluid pressure upstream of the inlet port 1 8 is zero at this time, the escaping of the pressurized fluid to the fluid reservoir through the fluid passage 24 can be effectively prevented by the check valve 26.

Therefore, upon the valve member 1 78 abuting the resilient member 236 and thereby moving the power piston, since the pressurized fluid in the accumulator 300 is supplied to the pressure chamber 11 4 through the check valve 330, the channel 148, the passage 174, the annular groove 1 64 and the cut-out recess 165, the boost pressure can be built up in the pressure chamber 11 4 in spite of damage of the fluid source.

Fig. 3 shows another embodiment of the power booster according to the present invention. As seen from Fig. 3, the check valve 400 is designed different from the foregoing embodiment. Namely, the movable stem 402 having a round end 404 is placed in the cone-shaped groove 352 at the round end 404. By the motion of the power piston 112, the round end 404 of the stem 402 is released from the cone-shaped groove 352 and pushes up the ball member 350 against the spring force of the set spring 356.

Though the present invention has been described in detail with the specific embodiment as illustrated hereabove, the invention can be embodied with various modifications for respective elements consisting the specific embodiment. Therefore, the various modifications of the embodiment as far as which can be expected from the principle of the invention should be understood as involved in the scope of the invention.

Claims (14)

1. A hydraulic power booster in a brake system for an automotive vehicle comprising: a booster housing communicating with a working fluid source and introducing therefrom a working fluid, which working fluid is recirculated through a hydraulic circuit via said booster housing; a power piston slidably disposed within said booster housing and movable along the longitudinal axis pf said housing, said power piston being connected with a brake master cylinder which distributes pressurized fluid to each'of individual wheel cylinders, said power piston defining a pressure chamber within said booster housing where a boost pressure is built up; a valve member slidably disposed within said power piston and cooperative with said power piston for connecting or disconnecting between said pressure chamber and said fluid source corresponding to application and release of a brake pedal;; a pressure accumulating means communicating with said fluid source to be supplied therefrom the working fluid and for accumulating means being communicating with said booster housing for supplying the pressurized working fluid to said booster housing; and a check valve means interpositioned between said booster housing and said pressure accumulating means for normally interrupting fluid flow from said accumulating means to said booster housing and for opening in response to motion of said power piston to permit fluid flow from said pressure accumulating means to said booster housing..

2. A hydraulic power booster in a brake system for an automotive vehicle comprising: à booster housing having an internal cham bdr therein and communicating with a working fluid source for recirculating the working fluid therethrough; a power piston disposed within said booster housing and slidable along the longitudinal axis thereof, said power piston being cooperated with a brake master cylinder in the hydraulic brake system for transmitting thereto a boost pressure built up in the booster, which power piston defining a pressure chamber in the booster housing to build up the boost pressure therein;; a valve member slidaly disposed within said power piston and cooperating with a brake pedal, which valve member being moved in response to application and release of the brake pedal for connecting or disconnecting between the pressure chamber and the fluid source; a pressure accumulating means for receiving the working fluid from said fluid source and accumulating the fluid pressure therein, which accumulating means communicating with the booster housing for supplying thereto a pressurized fluid; a first check valve means interpositioned between said pressure accumulating means and said pressure chamber so as to normally interrupting communication therebetween and establishing communication therebetween when the valve member is moved to the applied position exceeding a predetermined value; and a second check valve means interpositioned between said fluid source and said booster housing, whjch second valve member permitting fluid flow from said fluid source to said booster housing and preventing the fluid from flowing in a reverse direction.

3. A hydraulic power booster in a brake system for an automotive vehicle comprising: - a cylindrical booster housing; a a cylindrical power piston disposed within the booster housing; a valve member disposed within said power piston and slidable along the longitudinal axis thereof; a primary hydraulic circuit connecting said booster housing with a fluid source and recirculating the working fluid therethrough via the booster housing; a pressure accumulating means provided in substantially parallel to the booster housing with respect to said primary hydraulic circuit and receiving the working fluid from said fluid source for accumulating the fluid pressure therein; a secondary hydraulic circuit connecting said booster housing and said pressure accu mulating means;; a first check valve means provided within said secondary hydraulic circuit for normally interrupting communication between the accumulating means and the booster housing; a second check valve provided upstream of the booster housing and the accumulating means in said primary hydraulic circuit for permitting fluid flow from the fluid source to said booster and said accumulating means' and preventing the fluid from flowing froni the booster housing and the accumulating means to said fluid source.

4. A booster as set forth in claim 1, 2 or 3, wherein said pressure accumulating means is hydropneumatically operable to accumulate the hydraulic pressure therein.

5. A booster as set forth in claim 4, wherein said pressure accumulating means comprises an accumulator housing defining therein a chamber and a movable valve member disposed within said chamber and deviding said chamber into first and second sections, said first section being communicated with said fluid source for introducing therefrom the working fluid and said second section being filled with a gas.

6. A booster as set forth in claim 5, wherein said accumulating means housing is integrally formed with said booster housing and said check valve means disposed between said accumulating means and the booster housing is located within the booster housing so that it opposes to the hollow interior of said bqoster housing.

7. A booster as set forth in claim 6, wherein said check valve means comprises a thrusting member disposed within a radial opening formed in the booster housing and movable along the longitudinal axis thereof, said thrusting member has a head portion normally seating onto a valve seat at one end thereof, the other end of said valve member is received in a groove formed in the power piston so that it can be pushed up corre-.

sponding to the piston motion to release the head portion thereof from said valve seat.

8. A booster as set forth in claim 3, wherein said first and second check valve means are respectively formed in the booster housing, which first valve means comprises a movable member diposed within a radial opening extending in the booster housing and movable along the longitudinal axis thereof, said movable member has a head portion normally seating onto a valve seat for closing the open end of the fluid passage communicating the accumulating means and the booster housing, which movable member being operable to move to release the head portion thereof from said valve seat in response to the motion of the power piston, and said second check valve means comprises a movable member movably disposed within an opening formed within the booster housing and having a head portion normally seated to a valve seat, said movable member of said second valve being responsive to the fluid pressure applied thereto in the direction from the fluid source to the pressure chamber to open the valve for permitting the fluid flow therethrough.

9. A hydraulic power booster in a brake system for an automotive vehicle comprising: a cylindrical booster housing having inlet and outlet ports respectively communicating with a fluid source to be supplied therefrom and to drain thereto the working fluid; a cylindrical power piston slidably disposed within said booster housing and movable along the longitudinal axis of said booster housing, said power piston defining a pressure chamber to build up a boost force therein upon applying brake, said power piston being connected with a brake system master cylinder for transmitting the boost force thereto;; a cylindrical valve member slidably disposed within said power piston and connected with a brake pedal through a mechanical linkage, said valve member being movable along the longitudinal axis of said power piston between the first and second positions, where, in first position, said valve member interrupting communication between said inlet port and said pressure chamber and, in second position, establishes communication therebetween; a hydropneumatic accumulator incorporated with said booster housing and communicated with said booster housing and said fluid source, said accumulator receiving fluid from said fluid source and thus accumulating fluid pressure therein and supplying the pressurized fluid therefrom to said booster housing;; a first check valve interpositioned between said booster housing and said accumulator, which valve being normally in a closed position and opened in response to the motion of said power piston in the direction applying brake boost force to said master cylinder; and a second valve interpositioned between said fluid source and said booster housing and said accumulator, which second valve permitting fluid flowing therethrough in the direction from the fluid source to said booster housing and said accumulator and restricting fluid flowing in a reverse direction.

10. A booster as set force in claim 9, wherein said accumulator comprises an accumulator housing integrally formed with said booster housing and a plunger member slidably disposed within said accumulator housing and movable along the longitudinal axis thereof, said plunger member defining in said accumulator housing first and second chambers, said first chamber is filled with gas and said second chamber is admitted the working fluid from said fluid source.

11. A booster as set force in claim 9 or 10, wherein said first valve comprises a substantially cylindrical valve housing having an longitudinal opening axially extending along the longitudinal axis thereof, a thrusting member disposed within said longitudinal opening and movable between first and second position, a fluid passage formed within said valve housing and connecting said booster housing and said accumulator and a valve element normally urged onto one end of said fluid passage for closing the end of the passage, said valve element is incooperated with said thrusting member and moved to open the end of passage for permitting the fluid flowing therethrough when said thrusting member is in said second position, and said thrusting member is cooperated with said power piston so that said thrusting member is moved from said first position to said second position upon said power piston moving in applying position of brake.

1 2. A booster as set forth in claim 11, wherein a means for cooperating said power piston and said thrusting member is a groove formed on the outer periphery thereof, the thrusting member is positioned in said groove at the end remote from said vlave element.

1 3. A booster as set forth in claim 9 or 10, wherein said first and second valves are respectively incorporated with in said booster housing.

14. A booster as set forth in claim 13, wherein said second valve member is communicated with said fluid source via a fluid passage formed within said booster housing, said fluid passage is branched upstream of said second valve, which branched passage is provided with a relief valve for relieving the working fluid when the fluid pressure applied to the booster exceed a predetermined value.