Hydraulic brake system

In a hydraulic brake system of the present invention, the communication between a power chamber 30 of a hydraulic booster 2 and wheel cylinders (WCY) 58, 59 is allowed by a switching valve 64 when fluid pressure of an accumulator (ACC) 46 exceeds predetermined pressure. During braking operation, the fluid pressure of the ACC 46 introduced in the power chamber 30 is introduced directly to the WCYs 58, 59, thereby rapidly actuating brakes and thus improving the response. When the fluid pressure of the ACC 46 is less than the predetermined pressure, the communication between a fluid chamber 57 of a master cylinder (MCY) 3 and the WCYs 58, 59 is allowed by the switching valve 64. During braking operation, MCY pressure developed by a MCY piston 53 operated by an input shaft 21 through a power piston 10 is introduced into the WCYs 58, 59 through the switching valve 64. The brakes can securely work even when the fluid pressure of the ACC 46 is less than the predetermined pressure.

BACKGROUND OF THE INVENTION 
The present invention relates to a hydraulic brake system using a hydraulic 
booster which boosts leg-power exerted on a brake pedal to a predetermined 
value, in which pressurized fluid introduced into a power chamber of the 
hydraulic booster is introduced into brake cylinders in order to actuate 
brakes, and more particularly to a hydraulic brake system in which master 
cylinder pressure developed in a master cylinder is supplied to brake 
cylinders through a switching valve when fluid pressure drops, thereby 
ensuring positive operation of brakes. 
Sometimes employed in a vehicle is a hydraulic brake system which uses a 
hydraulic booster, which boosts leg-power exerted on a brake pedal to a 
predetermined value by pressurized fluid. In such a brake system, the 
pressurized fluid introduced into a power chamber of the hydraulic booster 
is introduced into brake cylinders in order to actuate brakes. Such a 
brake system can provide sufficient braking force with small leg-power on 
the brake pedal, thereby ensuring positive operation of the brakes and 
reducing the driver's labor. 
In such a hydraulic brake system, it is desired to ensure positive 
operation of the brakes even when the fluid pressure drops. As one of 
conventional hydraulic brake systems which can ensure the positive 
operation of the brakes even when the fluid pressure drops, proposed in 
Japanese Unexamined Patent Publication No. 64-47659 is a hydraulic brake 
system which operates brakes by supplying fluid pressure developed in a 
hydraulic booster by a switching valve to wheel cylinders during normal 
operation, and ensure positive operation of the brakes by supplying master 
cylinder pressure developed in a master cylinder to wheel cylinders when 
fluid pressure drops. 
In the hydraulic brake system disclosed in this publication, the operation 
of the switching valve is controlled by the fluid pressure introduced into 
a power chamber of the hydraulic booster during the braking operation. To 
be described in detail, the system has a piston disposed in the switching 
valve for controlling the operation of the switching valve and the piston 
is provided with a large-diameter portion and a small-diameter portion. 
When the braking operation is not performed, the piston is set in a 
position where the communication between the power chamber of the 
hydraulic booster and the wheel cylinders is interrupted and the 
communication between a fluid chamber of the master cylinder and the wheel 
cylinders is allowed, while when the braking operation is performed and 
the pressurized fluid is thereby introduced into the power chamber, the 
fluid pressure in the power chamber is applied to the large-diameter 
portion of the piston so as to set the piston in a position where the 
communication between the fluid chamber of the master cylinder and the 
wheel cylinders is interrupted and the communication between the power 
chamber of the hydraulic booster and the wheel cylinders is allowed. 
However, in the conventional hydraulic brake system, the switching valve is 
set to interrupt the communication between the power chamber of the 
hydraulic booster and the wheel cylinders and allow the communication 
between the fluid chamber of the master cylinder and the wheel cylinders 
when the braking operation is not performed. 
Therefore, when the braking operation is performed for normal braking under 
conditions of normal fluid pressure, the pressurized fluid is not supplied 
to the wheel cylinders until the switching valve is switched even after 
the fluid pressure is introduced into the power chamber of the hydraulic 
booster. Accordingly, the response of this system is not necessarily good. 
In addition, the direction, in which the fluid pressure is exerted on the 
large-diameter portion of the piston of the switching valve to switch the 
switching valve, opposes the direction, in which the fluid pressure of the 
pressurized fluid to be supplied to the wheel cylinders is exerted on the 
small-diameter portion, thereby causing loss in the fluid pressure. This 
is also a reason making the response insufficient. 
When the fluid pressure drops, the fluid pressure in the master cylinder is 
supplied to the wheel cylinders through the switching valve. The master 
cylinder pressure is exerted on the piston in such a manner that the 
piston is moved in such a direction as to interrupt the communication 
between the fluid chamber of the master cylinder and the wheel cylinders. 
As the force of a spring is set to large in order to prevent the moving of 
the piston, the piston of the switching valve is difficult to move quickly 
when the hydraulic booster is actuated under conditions of normal fluid 
pressure, thereby further making the response worse. It is therefore not 
simply solved by just setting the force of the spring larger. Moreover, it 
is quite difficult to determine the force of the spring biasing a piston 
and a pressure receiving area of fluid pressure of the piston in such a 
manner as to securely and rapidly move the piston of the switching valve 
in the direction that allows the communication between the power chamber 
of the hydraulic booster and the wheel cylinder and interrupts the 
communication between the fluid chamber of the master cylinder and the 
wheel cylinders when the hydraulic booster is actuated under conditions of 
normal fluid pressure, and securely not to move the piston in the 
direction that allows the communication between the power chamber of the 
hydraulic booster and the wheel cylinder and interrupts the communication 
between the fluid chamber of the master cylinder and the wheel cylinders 
when the fluid pressure drops. 
There is another problem that components of the switching valve such as a 
seal of the piston are inferior in durability because the piston of the 
switching valve moves in such a direction as to increase the volume of the 
fluid chamber of the master cylinder so as to increase the pedal stroke 
for normal braking and the piston moves every time the hydraulic booster 
is actuated. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a hydraulic brake 
system which can positively operate brakes under condition of low fluid 
pressure while providing good durability during normal braking under 
conditions of normal fluid pressure. 
It is another object of the present invention to provide a hydraulic brake 
system which can allow reduced pedal stroke while providing good 
durability. 
In order to achieve this object, the present invention provides a hydraulic 
brake system comprising: a fluid pressure source normally developing fluid 
pressure exceeding predetermined pressure; an input shaft which is 
operated when braking operation is performed; a hydraulic booster having a 
control valve controlled by the input shaft, a power chamber into which 
the fluid pressure is introduced from the fluid pressure source when the 
braking operation is performed, and a power piston which is actuated by 
the fluid pressure in the power chamber or by the input shaft, the fluid 
pressure being discharged from the power chamber by the control valve when 
the braking operation is not performed, the fluid pressure corresponding 
to the operating force being introduced into the power chamber when the 
braking operation is performed, and the hydraulic booster outputting 
according to the operation of the power piston by the fluid pressure in 
the power chamber; a master cylinder having a master cylinder piston which 
is interlocked with the power piston by the output of the hydraulic 
booster, the master cylinder developing master cylinder pressure in a 
fluid chamber thereof by the operation of the master cylinder piston; 
brake cylinders developing braking forces; and a switching valve 
selectively switching the brake cylinders to communicate with the power 
chamber of the hydraulic booster or with the fluid chamber of the master 
cylinder, wherein the switching valve is controlled by the fluid pressure 
of the fluid pressure source, and communicates the power chamber of the 
hydraulic booster with the brake cylinders when the fluid pressure of the 
fluid pressure source exceeds predetermined pressure or communicates the 
fluid chamber of the master cylinder with the brake cylinders when the 
fluid pressure of the fluid pressure source is less than the predetermined 
pressure. 
In addition, in the present invention the switching valve comprises a valve 
controlling the communication or interruption between the power chamber 
and the brake cylinder, and a switching control piston valve which is 
exerted with the fluid pressure when the fluid pressure of the fluid 
pressure source exceeds the predetermined pressure and thus opens the 
valve to allow the communication between the power chamber and the brake 
cylinders and to interrupt the communication between the fluid chamber of 
the master cylinder and the brake cylinders or which closes the valve to 
interrupt the communication between the power chamber and the brake 
cylinders and to allow the communication between the fluid chamber of the 
master cylinder and the brake cylinder. 
In the hydraulic brake system of the present invention as structured above, 
the switching valve allows the communication between the power chamber of 
the hydraulic booster and the brake cylinders whenever the fluid pressure 
of the fluid pressure source exceeds the predetermined pressure. 
Therefore, during braking operation, the fluid pressure introduced into 
the power chamber is directly introduced to the brake cylinder, thereby 
rapidly actuating the brakes. Accordingly, the response of the hydraulic 
brake system is improved. In addition, because the switching valve is 
controlled by the fluid pressure in the fluid pressure source, the fluid 
pressure, to be introduced to the brake cylinder, in the power chamber of 
the hydraulic booster never produces loss, thereby further improving the 
response of the hydraulic brake system. 
Moreover, the switching valve allows the communication between the fluid 
chamber of the master cylinder and the brake cylinders when the fluid 
pressure in the fluid pressure source is less than the predetermined 
pressure. Since the input shaft thereby operates the master cylinder 
piston through the power piston when the braking operation is performed, 
the master cylinder develops master cylinder pressure which is then 
introduced into the brake cylinders through the switching valve. As a 
result of this, the brakes are securely actuated during the braking 
operation even when the fluid pressure in the fluid pressure source 
becomes less than the predetermined pressure. 
Still other objects and advantages of the invention will in part be obvious 
and will in part be apparent from the specification. 
The invention accordingly comprises the features of construction, 
combinations of elements, and arrangement of parts which will be 
exemplified in the construction hereinafter set forth, and the scope of 
the invention will be indicated in the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a sectional view showing one embodiment of a hydraulic brake 
system in accordance with the present invention and FIG. 2 is a partly 
enlarged sectional view of a hydraulic booster of the hydraulic brake 
system shown in FIG. 1. 
As shown in FIG. 1 and FIG. 2, the hydraulic brake system of this 
embodiment comprises a brake pressure-producing device 1 which includes a 
hydraulic booster 2, a master cylinder (hereinafter, sometimes referred to 
as "MCY") 3 actuated by the output of the hydraulic booster 2, the 
hydraulic booster 2 and the master cylinder 3 being integrated, and a 
housing 4 which is common to the hydraulic booster 2 and the MCY 3. 
As for the hydraulic booster 2, the housing 4 is provided with a first hole 
5 opening toward the right hand side in FIG. 1, a second hole 6 
continuously formed at the left side of the first hole 5, the diameter of 
the second hole 6 being smaller than that of the first hole 5, and a third 
hole 7 continuously formed at the left side of the second hole 6 and 
having a closed left end, the diameter of the third hole 7 being smaller 
than that of the second hole 6. The first, second, and third holes 5, 6, 7 
are integrally formed as a stepped hole. The right end of the first hole 5 
is hermetically closed by a plug 8 which is in contact with the stepped 
portion between the first hole 5 and the second hole 6 and is fixed to the 
housing 4 by a nut 9 threaded into the housing 4. 
Disposed within the second and third holes 6, 7 and extending across them 
is a power piston 10 of the hydraulic booster 2 which is structured as a 
stepped piston comprising a large-diameter portion 10a at the right side 
thereof having substantially the same diameter of the second hole 6 and a 
small-diameter portion 10b at the left side thereof having substantially 
the same diameter of the third hole 7. The large-diameter portion 10a is 
sealed with O-rings and allowed to slide in the second hole 6 and the 
small-diameter portion 10b is sealed by a cup packing 11 and allowed to 
slide in the third hole 7. In this case, the cup packing 11 seals fluid 
only in one direction so as to prevent fluid from flowing from the second 
hole 6 toward the third hole 7 while allowing fluid to flow from the third 
hole 7 toward the second hole 6 between the small-diameter portion 10b of 
the power piston 10 and the inner wall of the third hole 7 of the housing 
4. 
The power piston 10 is provided with a fourth hole 12 opening at the right 
end of the power piston 10 and a fifth hole 13 formed continuously from 
the left end of the forth hole 12 and having a closed left end thereof, 
the diameter of the fifth hole 13 being smaller than that of the fourth 
hole 12. The fourth and fifth holes 12, 13 are integrally formed as a 
stepped hole. Hermetically disposed in the fifth hole 13 is a cylindrical 
valve seat member 14 (not shown) which is fixed to the power piston 10 in 
such a manner that a flange 14a of the valve seat member 14 comes in 
contact with the stepped portion between the forth hole 12 and the fifth 
hole 13 by a nut 16 threaded into the right end of the power piston 10 
through a cylindrical fixing member 15 inserted into the fourth hole 12. 
The cylindrical fixing member 15 is slidably fitted onto the circumference 
of a cylindrical projection 8a extending in the axial direction of the 
plug 8. 
Slidably inserted into the fifth hole 13 is a valve body 18 supporting a 
valve ball 17. The valve body 18 is always biased by a spring 19 in such a 
direction that the valve ball 17 is seated in a first valve seat 14b of 
the valve seat member 14. The valve seat member 14 has an axial hole 14c 
in which a cylindrical member 20 is disposed. The cylindrical member 20 
has a second valve seat 20a formed at the front end thereof in which the 
valve ball 17 is seated. The cylindrical member 20 is inserted into an 
axial hole of a cylindrical stopper 22 and is fixed to the cylindrical 
stopper 22 by a spring 23 compressed between the valve seat member 14 and 
the cylindrical member 20, the cylindrical stopper 22 being securely 
fitted onto the circumference of an input shaft 21 at the left end of the 
input shaft 21. The input shaft 21 and the cylindrical stopper 22 
penetrate the cylindrical portion 8a of the plug 8 through the axial hole 
8b thereof and the rear end of the input shaft 21 is connected to a brake 
pedal which is not shown. The rearmost positions for the cylindrical 
member 20 and the input shaft 21 are defined by that a flange 22a of the 
cylindrical stopper 22 comes in contact with a front end 8c of the 
cylindrical projection 8a of the plug 8. 
Slidably inserted between the outer peripheries of the input shaft 21 and 
the cylindrical stopper 22 and the inner periphery of the axial hole 8b of 
the cylindrical projection 8a of the plug 8 is a cylindrical reaction 
piston 24. As shown in FIG. 3, the reaction piston 24 is provided with a 
first flange 24a and a second flange 24b at the left end thereof in FIG. 
3. Since the flange 22a of the cylindrical stopper 22 can come in contact 
with the left end of the first flange 24a, the left end of the first 
flange 24a at this point functions as a stopper 24c for preventing further 
backward movement of the cylindrical stopper 22 relative to the reaction 
piston 24. 
The left end of the second flange 24b functions as an engagement member 24d 
which engages with a stepped portion 15a of the cylindrical fixing member 
15 when the reaction piston 24 retreats to a predetermined extent relative 
to the power piston 10. The right end 24e of the reaction piston 24 can 
come in contact with a stepped portion 21a of the input shaft 21. 
Compressed between the second flange 24b of the reaction piston 24 and the 
cylindrical fixing member 15 is a spring 25 whereby the second flange 24b 
of the reaction piston 24 is normally in contact with the flange 14a of 
the valve seat member 14. 
The housing 4 is further provided with an inlet 26 through which 
pressurized fluid is introduced and a path 27 allowing the communication 
between the inlet 26 and the second hole 6 and the power piston 10 is 
provided with an annular groove 28, which communicates with the path 27, 
formed in the circumference of the power piston 10 and a path 29 allowing 
the communication between the annular groove 28 and the fifth hole 13 at 
the valve body 18 side of the valve seat member 14. The inlet 26, the path 
27, the annular groove 28, and the path 29 form together a fluid pressure 
supply passage. 
A space in the second hole 6 defined between the plug 8 and the right end 
of the power piston 10 is a power chamber 30 which always communicates 
with the axial hole 14c of the valve seat member 14. The flange 22a of the 
cylindrical stopper 22 and the first and second flange 24a, 24b of the 
reaction piston 24 are positioned in the power chamber 30. Formed between 
the outer periphery of the cylindrical projection 8a of the plug 8 and the 
inner periphery of the cylindrical fixing member 15 is an axial groove 
(not shown) so as to allow free flow of hydraulic fluid on the both sides 
of the cylindrical fixing member 15. The power chamber 30 always 
communicates with a first outlet 32 through a path 31 formed in the 
housing 4, and the first outlet 32 always communicates with wheel 
cylinders (hereinafter, sometimes referred to as "WCYs") 33, 34 relating 
to one of two braking circuits constituting the brake system. 
Moreover, the axial hole 20b of the cylindrical member 20 opening to both 
the right and left sides always communicates with a discharge port 41 
through an axial path 35 and a radial path 36 which are formed in the 
input shaft 21, an annular groove 37 and a radial path 38 which are formed 
in the plug 8, an annular space 39 formed between the plug 8 and the 
housing 4, and an axial passage 40 formed in the housing 4. The discharge 
port 41 always communicates with a booster reservoir 42 for the hydraulic 
booster. 
Further, a hydraulic circuit connecting the inlet 26 and the booster 
reservoir 42 is provided with a hydraulic pump 44 driven by a motor 43 and 
an accumulator (hereinafter, sometimes referred to as "ACC) 46 at the 
discharge side of the hydraulic pump 44 via a check valve 45. 
Predetermined pressure is always stored in the ACC 46 by the discharge 
pressure of the hydraulic pump 44. 
In the inoperative state i.e. when the brake pedal is not pedaled, the 
valve ball 17, the valve seat member 14, and the front end 20a (the second 
valve seat) of the cylindrical member 20 are in positions as shown in FIG. 
1 and FIG. 2. That is, the valve ball 17 is seated in the first valve seat 
14b of the valve seat member 14 and the second valve seat 20a of the 
cylindrical member 20 is apart from the valve ball 17. In this state, 
while the communication between the path 29 always communicating with the 
inlet 26 and the axial hole 14c of the valve seat member 14 is 
interrupted, the communication between the axial hole 14c of the valve 
seat member 14 and the axial hole 20b of the cylindrical member 20 always 
communicating with the discharge port 41 is allowed. Therefore, in the 
inoperative state, the power chamber 30 is shut off from the pump 44 and 
the ACC 46 and communicates with the booster reservoir 42 so that 
pressurized fluid is not supplied into the power chamber 30. 
In the operative state i.e. when the brake pedal is pedaled, the input 
shaft 21 moves forward so that the valve ball 17 is seated in the second 
valve seat 20a of the cylindrical member 20 and the valve ball 17 is 
parted from the first valve seat 14b of the valve seat member 14. In this 
state, therefore, the communication between the path 29 and the axial hole 
14c of the valve seat member 14 is allowed while the communication between 
the axial hole 14c of the valve seat member 14 and the axial hole 20b of 
the cylindrical member 20 is interrupted. That is, during the braking 
operation, the power chamber 30 communicates with the pump 44 and the ACC 
46 and is shut off from the booster reservoir 42 so that pressurized fluid 
is supplied into the power chamber 30. 
In this manner, the valve ball 17, the first valve seat of the valve seat 
member 14 and the second valve seat of the cylindrical member 20 
constitute together a control valve 47 for the hydraulic booster 2 for 
selectively switching the power chamber 30 to communicate with the pump 44 
and the ACC 46 or with the booster reservoir 42. 
Furthermore, the power chamber 30 always communicates with a chamber 49 
facing the left end of the valve body 18 through an axial path 48 formed 
in the power piston 10, and always communicates with an annular chamber 51 
formed between the inner periphery of the second hole 6 of the housing 4 
and the outer periphery of the small-diameter portion 10b of the power 
piston 10 through the path 48 and a radial path 50, extending from the 
path 48, formed in the power piston 10. The annular chamber 51 
accommodates a return spring 52 which always biases the power piston 10 in 
a direction toward the inoperative position. 
As for the MCY 3, this MCY 3 has the same structure as a general 
conventional single MCY. That is, a MCY piston 53 is slidably inserted 
into the third hole 7 of the housing 4. In addition, a MCY reservoir 54 is 
mounted on the housing 4 and the housing 4 is provided with a brake fluid 
supply port 55 and a compensating port 56 to communicate the MCY reservoir 
54 with the third hole 7. Moreover, a fluid chamber 57 is defined in the 
third hole 7 by MCY piston 53. Mounted on the front end of the MCY piston 
53 is a cup packing 60. When the cup packing 60 is in the inoperative 
position in the right side of the opening end of the compensating port 56, 
the fluid chamber 57 communicates with the MCY reservoir 54 so that MCY 
pressure is not developed in the fluid chamber 57. When the cup packing 60 
of the MCY piston 53 then moves toward the left side of the opening end of 
the compensating port 56 and play in stroke for the WCYs 58, 59 is 
canceled, the MCY pressure is developed. 
Moreover, in the brake pressure-producing device 1 of the hydraulic brake 
system of this embodiment, the effective receiving area of an annular 
stepped portion 10c between the large-diameter portion 10a and the 
small-diameter portion 10b of the power piston 10 is set to be dimensions 
given by subtracting the effective receiving area of the small-diameter 
portion 10b of the power piston 10 by the cup packing 11 from the 
effective receiving area of the power piston 10 to which the fluid 
pressure in the power chamber 30 is exerted while the effective receiving 
area of the small-diameter portion 10b of the power piston 10 by the cup 
packing 11 is set to be equal to the effective receiving area of the MCY 
piston 53 to which the MCY pressure is exerted. That is, the substantial 
receiving area of the power piston 10 to which the fluid pressure in the 
power chamber 30 is exerted is set to be equal to the effective receiving 
area of the MCY piston 53 to which the MCY pressure in the fluid chamber 
57 of the MCY 3 is exerted. 
Further, the MCY piston 53 is always biased to the right, i.e. in the 
direction toward the inoperative position by a return spring 61 and an 
aligning rod 62 is disposed between the power piston 10 and the MCY piston 
53 whereby the both pistons 10, 53 are aligned and are allowed to be 
interlocked with each other. 
The annular chamber 51 of the hydraulic booster 2 communicates, through a 
second outlet 63 formed in the housing 4, with a first port 64a of a 
switching valve 64 consisting of a two position four-way valve while a 
second port 64b of the switching valve 64 communicates with the WCYs 58, 
59 relating to the other one of the two braking circuits. The fluid 
chamber 57 of the MCY 3 communicates with a third port 64c of the 
switching valve 64 while a fourth port 64d of the switching valve 64 
communicates with a stroke simulator 65 for ensuring the stroke of the MCY 
piston 53. 
The switching valve 64 can switch between a first position I where the 
first port 64a and the second port 64b are in communication and the third 
port 64c and the fourth port 64d are in communication and a second 
position II where the first port 64a is shut off from the other ports and 
the third port 64c and the second port 64b are in communication. The fluid 
pressure of the ACC 46 is introduced as pilot pressure into a pilot 
pressure inlet 64e of the switching valve 64 through pilot pressure path 
66, the pilot pressure when exceeding predetermined pressure shifts the 
switching valve 64 to the first position I. The switching valve 64 is 
biased by the spring force of a spring 67 which shifts the switching valve 
64 to the second position II when the pilot pressure is less than the 
predetermined pressure. 
FIG. 4 is a view of a concrete example of the switching valve 64. 
As shown in FIG. 4, the switching valve 64 comprises a housing 68 and a 
switching control piston valve 70 which is hermetically and slidably 
disposed, by two O-rings 71, 72, in an axial hole 69 formed in the housing 
68. The switching control piston valve 70 is provided with an annular 
groove 73 formed in the outer periphery thereof between the O-rings 71 and 
72. 
Also hermetically inserted into the axial hole 69 is a valve seat member 74 
having a valve seat 74a, the valve seat member 74 being fixed by a plug 75 
threaded into the housing 64. A valve ball 76 is disposed in the inner 
hole of the valve seat member 74 in such a manner that the valve ball 76 
can be seated in the valve seat 74a. The valve ball 76 is supported by a 
valve body 78 which is hermetically inserted into a hole 77 of the plug 
75. The valve ball 76 is always biased in such a direction as to be seated 
in the valve seat 74a by the spring force of a spring 79 compressed 
between the valve body 78 and the plug 75. 
Projecting from the end of the switching control piston valve 70, facing 
the valve ball 76, toward the valve ball 76 is a valve opening pin 80 
pushing the valve ball 76 in such a direction as to detach the valve ball 
76 from the valve seat 74a. The switching control piston valve 70 is 
always biased in such a direction as to part the valve opening pin 80 from 
the valve ball 76 by the spring force of a spring 81 compressed between 
the switching control piston valve 70 and the valve seat member 74. These 
springs 79 and 81 constitute together the spring 67 for shifting the 
switching valve 64 to the second position II. 
The first port 64a formed in the housing 64 always communicates with the 
inner hole of the valve seat member 74, in which the valve ball 76 and the 
valve body 78 are accommodated, through a radial path 82 formed in the 
valve seat member 74. The second port 64b formed in the housing 64 always 
communicates with the axial hole 69, in which the valve opening pin 80 is 
positioned, through a radial path 83 formed in the valve seat member 74. 
In addition, the third port 64c and the fourth port 64d formed in the 
housing 64 communicate with the axial hole 69 within a range where the 
switching control piston valve 70 slides. The pilot pressure inlet 64e 
formed in the housing 64 faces the end opposite to the end of the 
switching control piston valve 70 from which the valve opening pin 80 
projects. 
When fluid pressure stored in the ACC 46 is less than the predetermined 
pressure, as shown in FIG. 4, the switching control piston valve 70 is 
biased to the right by the spring force of the spring 81 so as to come in 
contact with the right end of the axial hole 69 of the housing 68. When 
the switching control piston valve 70 is in this position, the valve 
opening pin 80 is largely parted from the valve ball 76 so that the valve 
ball 76 is seated in the valve seat 74a and the first port 64a is thus 
shut off from the other ports. In addition, the third port 64c 
communicates with the second port 64b and the fourth port 64d is shut off 
from the other ports. That is, the switching valve 64 is set in the second 
position II. 
When the fluid pressure stored in the ACC 46 exceeds the predetermined 
pressure, as shown in FIG. 5, the switching control piston valve 70 is 
shifted to the left against the spring force of the spring 81 by the pilot 
pressure, developed by the fluid pressure of the ACC 46 and introduced 
into the switching control piston valve 70 through the pilot pressure 
inlet 64e, so that the valve opening pin 80 pushes and detaches the valve 
ball 76 from the valve seat 74a. When the switching control piston valve 
70 is in this position, the first port 64a communicates with the second 
port 64b because the valve ball 76 is apart form the valve seat 74a. In 
addition, the third port 64c and the fourth port 64d are positioned 
between the two O-rings 71 and 72 of the switching control piston valve 70 
and the O-ring 72 is positioned between the second port 64b and the third 
port 64c so that the third port 64c is shut off form the second port 64b 
and communicates with the fourth port 64d through the annular groove 73. 
That is, the switching valve 64 is set in the first position I. 
The plug 75 is provided with a MCY pressure inlet 64f communicating with 
the hole 77. Therefore, when the MCY pressure is introduced into the axial 
hole 69 through the third port 64c so that the MCY pressure is exerted on 
the valve ball 76 in such a manner as to part the valve ball 76 from the 
valve seat 74a while the switching valve 64 is in the second position II 
as shown in FIG. 4, the MCY pressure is also exerted on the valve body 78 
through the MCY pressure inlet 64f in such a direction that the valve ball 
76 is seated in the valve seat 74a, thereby preventing the valve ball 76 
from being parted from the valve seat 74a. 
In the brake pressure-producing device 1 of the hydraulic brake system of 
this embodiment as structured above, normally, the motor 43 is driven to 
operate the hydraulic pump 44 so that fluid pressure exceeding the 
predetermined pressure is stored in the ACC 46. In this state, the fluid 
pressure in the ACC 46 is introduced into the pilot pressure inlet 64e 
through the pilot pressure path 66 whereby the switching valve 64 is set 
in the first position I as shown in FIG. 1 and FIG. 5. As a result of 
this, the WCYs 58, 59 communicate with the power chamber 30 through the 
annular chamber 51 of the hydraulic booster 2 and are shut off from the 
fluid chamber 57 of the master cylinder 3. 
In the inoperative state i.e. when the brake pedal is not pedaled, the 
input shaft 21 does not move forward and the control valve 47 of the 
hydraulic booster 2 is thus in the inoperative state as shown in FIG. 1 
and FIG. 2. Therefore, no pressurized fluid is supplied from the ACC 46 to 
the power chamber 30 so that the power piston 10 does not work and the 
hydraulic booster 2 does not output. Since the annular chamber 51 always 
communicates with the power chamber 30, no pressurized fluid is also 
supplied from the ACC 46 to the annular chamber 51 when the brake pedal is 
not pedaled. 
The right end 24e of the reaction piston 24 is apart form the stepped 
portion 21a of the input shaft 21 and the flange 22a of the cylindrical 
stopper 22 is apart form the stopper 24c of the first flange 24a and is 
therefore advanced from the stopper 24c. 
As the braking operation is performed by pedaling the brake pedal, the 
input shaft 21 and the cylindrical member 20 move forward to switch the 
control valve 47 as mentioned above. As a result of this, the power 
chamber 30 is shut off form the booster reservoir 42 and communicates with 
the ACC 46 so that the pressurized fluid is introduced from the ACC 46 
into the power chamber 30. When the pressure of the pressurized fluid 
introduced in the power chamber 30 grows to reach a level that can 
overcome the spring forces of the return springs 52 and 61, the power 
piston 10 moves forward by the pressurized fluid so that the hydraulic 
booster 2 is actuated and the MCY piston 53 moves forward, thereby 
developing MCY pressure in the fluid chamber 57. The pressurized fluid in 
the power chamber 30 is introduced into the WCYs 33, 34 relating to the 
one circuit and is introduced into the annular chamber 51 through the 
paths 48, 50. The pressurized fluid introduced into the annular chamber 51 
is further introduced into the WCYs 58, 59 relating to the other circuit 
through the second outlet 63 and the switching valve 64. In addition, the 
MCY pressure is introduced into the stroke simulator 65 via the switching 
valve 64, thereby ensuring the stroke of the master cylinder piston 53, 
i.e. the stroke of the power piston 10. 
Though the reaction piston 24 is shifted to the right relative to the power 
piston 10 and the input shaft 21 by the fluid pressure in the power 
chamber 30 against the spring force of the spring 25, the rear end 24e of 
the reaction piston 24 never reaches to the stepped portion 21a of the 
input shaft 21. The fluid pressure in the annular chamber 51, i.e. the 
fluid pressure in the power chamber 30 is exerted on the annular stepped 
portion 10c of the power piston 10 in the direction opposite to the 
direction exerting on the power piston 10. In this case, since the 
effective receiving area of the power piston 10 on which the fluid 
pressure in the power chamber 30 is substantially exerted is equal to the 
effective receiving area of of the MCY piston 53 where the MCY in the 
fluid chamber 57 is received as mentioned above while the power piston 10 
and the MCY piston 53 are interlocked with each other through the aligning 
rod 62, the fluid pressure in the power chamber 30 and the MCY pressure 
balance to be equal to each other. 
The pressurized fluid in the power chamber 30 is further introduced into 
the chamber 49 through the axial path 48 so that the valve body 18 is 
biased in the direction opposing the input of the input shaft 21 by the 
fluid pressure in the chamber 49. 
In the initial stage where the WCYs produce substantially no braking force 
wherein there is play in stroke for each WCY 34, 35; 58, 59, the right end 
24e of the reaction piston 24 does not come in contact with the stepped 
portion 21a of the input shaft 21 so that no force from the reaction 
piston 24 is exerted on the input shaft 21. Therefore, exerted on the 
input shaft 21 is force developed by the fluid pressure in the power 
chamber 30 and received by relatively small effective receiving areas of 
the cylindrical stopper 22 and the cylindrical member 20 and this force is 
transmitted as reaction force to a driver. 
As the reaction force becomes equal to the input force of the input shaft 
21, the valve ball 17 is seated in both the first valve seat 14b and the 
second valve seat 20a so that the power chamber 30 is shut off from the 
ACC 46 and the booster reservoir 42. As the input force of the input shaft 
21 further grows, the valve ball 17 is seated in the second valve seat 20a 
and the valve ball 17 is parted from the first valve seat 14b again so 
that further fluid pressure is supplied to the power chamber 30 with the 
result that the fluid pressure in the power chamber 30 further rises. 
After that, by repeating the seating of the valve ball 17 in the first 
valve seat 14b, as the input of the input shaft 21 grows, the fluid 
pressure in the power chamber 30 successively rises at the predetermined 
power ratio. 
While the respective WCYs 33, 34; 58, 59 are in play range of stroke, the 
right end 24e of the reaction piston 24 does not come in contact with the 
stepped portion 21a of the input shaft 21 so that the effective receiving 
area of the input shaft 21 on which the fluid pressure in the power 
chamber 30 is exerted is small and the power ratio at this point is 
therefore large. Accordingly, the output of the hydraulic booster 2 rises 
quite largely relative to the input force of the input shaft 21 at the 
large power ratio so that the hydraulic booster 2 performs the so-called 
jumping action. 
As the fluid pressure in the power chamber 30 further rises and the power 
piston 10 further moves forward so that the play in stroke for the 
respective WCYs 33, 34; 58, 59 is canceled, the WCYs 33, 34; 58, 59 
produce braking forces and the brakes thereby substantially work. In this 
state, the right end 24e of the reaction piston 24 comes in contact with 
the stepped portion 21a of the input shaft 21 by the risen fluid pressure 
in the power chamber 30 and the reaction piston 24 exerts force on the 
input shaft 21 with the biasing force by fluid pressure in the power 
chamber 30 in such a manner as to oppose the input force of the input 
shaft 21. Therefore, the reaction force exerted on the input shaft 21 
grows and the output of the hydraulic booster 2 rises relative to the 
input force of the =. input shaft 21 with a ratio smaller than that when 
the WCYs are in the play range and then the jumping action is finished. 
After that, since the reaction force grows, the hydraulic booster 2 boosts 
the input force of the input shaft 21 at a relatively small power ratio 
and the fluid pressure in the power chamber 30 becomes to correspond to 
the power ratio. Then, the pressurized fluid in the power chamber 30 is 
introduced into the respective WCYs 33, 34; 58, 59 so that the WCYs 33, 
34; 58, 59 produce braking forces which are large relative to the input 
force of the input shaft 21 and the brakes thereby work with the braking 
forces. 
As releasing the brake pedal to cancel the braking, the input shaft 21 and 
the cylindrical member 20 move to the right and the second valve seat 20a 
of the control valve 47 is parted from the valve ball 17 so that the power 
chamber 30 communicates with the booster reservoir 42. Therefore, the 
pressurized fluid in the power chamber 30 is discharged into the booster 
reservoir 42 through the axial hole 14c of the valve seat member 14, a 
space between the valve ball 17 and the second valve seat 20a, the axial 
hole 20b of the cylindrical member 20, the axial path 35 and radial path 
36 of the input shaft 21, the annular groove 37 and the radial path 38 of 
the plug 8, an annular space 39 between the plug 8 and the housing 4, the 
axial path 40 of the housing 4, and the discharge port 41. At this point, 
since the input shaft 21 largely retreats until the flange 22a of the 
cylindrical stopper 22 comes into contact with the stopper 24c of the 
reaction piston 24, the second valve seat 20a is largely parted from the 
valve ball 17. Therefore, the pressurized fluid in the power chamber 30 is 
rapidly discharged, thereby reducing the fluid pressure in the power 
chamber 30. 
As a result of the discharge of the pressurized fluid in the power chamber 
30, the pressurized fluid in the respective WCYs 33, 34; 58, 59 is also 
rapidly discharged into the booster reservoir 42 through the power chamber 
30 (the pressurized fluid in the WCY 58, 59 is discharged through the 
switching valve 64 and the annular chamber 51) and the power piston 10 
rapidly retreats by the spring force of the return spring 52, thereby 
rapidly canceling the braking. As a result of the retreat of the power 
piston 10, the MCY piston 53 also rapidly retreats so that the fluid 
chamber 57 of the MCY 3 communicates with the MCY reservoir 54. 
As the fluid pressure in the power chamber 30 is reduced to the 
predetermined pressure, the reaction piston 24 moves forward relative to 
the power piston 10 and the input shaft 21 by the spring force of the 
spring 25 and comes in contact with the flange 14a of the valve seat 
member 14 while the right end 24e of the reaction piston 24 is parted from 
stepped portion 21a of the input shaft 21. 
As the input shaft 21 further retreats until the cancellation of the 
braking is nearly finished, the flange 22a of the cylindrical stopper 22 
comes in contact with the front end 8c of the cylindrical projection 8a of 
the plug 8, thereby stopping the retreat of the input shaft 21 and the 
cylindrical member 20. At this point, the input shaft 21 and the 
cylindrical member 20 are in the rearmost positions. However, even when 
the input shaft 21 and the cylindrical member 20 stop retreating, the 
power piston 10, the reaction piston 24, the valve ball 17, and the valve 
seat member 14 continue to retreat. Therefore, the flange 22a of the 
cylindrical stopper 22 is parted from the stopper 24c of the reaction 
piston 24 while the valve ball 17 approaches the second valve seat 20a of 
the cylindrical member 20. 
As the right end of the power piston 10 comes in contact with the plug 8 as 
shown in FIG. 1 and FIG. 2, the power piston 10 stops retreating and is in 
the inoperative position and the MCY piston 53 also stops retreating and 
is in the inoperative position, thereby rapidly and completely canceling 
the braking. In this state, the valve ball 17 is very close to the second 
valve seat 20a so that the space between the valve ball 17 and the second 
valve seat 20a is so small, that is, the valve ball 17 is on the verge of 
sitting. Therefore, if the input shaft 21 and the cylindrical member 20 
move forward by pedaling the brake pedal, the valve ball 17 sits straight 
in the second valve seat 20a while the valve ball 17 is parted straight 
from the first valve seat 14b. That is, the play in stroke for performing 
the switching operation of the control valve 47 is significantly reduced, 
thereby rapidly actuating the brakes. 
In this manner, the brakes are rapidly actuated when the braking operation 
is performed, while the braking is rapidly canceled when the braking 
operation is canceled. In short, the brake pressure-producing device 1 has 
excellent response. 
On the other hand, as the fluid pressure in the ACC 46 becomes smaller than 
the predetermined pressure, the switching control piston valve 70 is moved 
to the right from the position shown in FIG. 5 by the spring force of the 
spring 81 and reaches the position shown in FIG. 4 so that the valve ball 
76 sits in the valve seat 64a. That is, the switching valve 64 is set in 
the second position II where the annular chamber 51 of the hydraulic 
booster 2 is shut off form the WCYs 58, 59 and the fluid chamber 57 of the 
master cylinder 3 communicates with the WCYs 58, 59. 
As the input shaft 21 moves forward by pedaling the brake pedal in this 
state, the second valve seat 20a of the cylindrical member 20 comes in 
contact with the valve ball 17 and the valve ball 17 is parted from the 
first valve seat 14b as mentioned above. However, since the fluid pressure 
in the ACC 46 is smaller than the predetermined pressure, the power piston 
10 does not move forward. As the input shaft 21 further moves forward, the 
valve body 18 holding the valve ball 17 comes in contact with the power 
piston 10 whereby the leg-power exerted on the brake pedal is transmitted 
to the power piston 10 through the input shaft 21, the cylindrical member 
20, the valve ball 17, and the valve body 18. After that, the power piston 
is moved forward with the leg-power in this manner. 
As a result of the forward movement of the power piston 10, the MCY piston 
53 also moves forward in the same manner as mentioned above so that MCY 
pressure is developed in the fluid chamber 57 and is introduced into the 
WCYs 58, 59 through the switching valve 64. The WCYs 58, 59 thus produce 
braking forces and the brakes relating to the other circuit are actuated 
with the leg-power. In this manner, the positive operation of the brakes 
is ensured even when the fluid pressure of the ACC 46 becomes smaller than 
the predetermined pressure. 
According to the hydraulic brake system of this embodiment, even when the 
fluid pressure of the ACC 46 becomes smaller than the predetermined 
pressure, the positive operation of the brakes is ensured because the MCY 
pressure in the MCY 3 is supplied to the WCYs 58, 59 by the switching 
valve 64 during the braking operation. 
Since the switching control of the switching valve 64 is performed with the 
fluid pressure of ACC 46 and the communication between the power chamber 
30 of the hydraulic booster 2 and the WCYs 58, 59 is allowed whenever 
fluid pressure exceeding the predetermined pressure is stored in the ACC 
46, the brakes can be actuated more rapidly than a case where the 
switching valve is controlled by the fluid pressure of the power chamber 
of the hydraulic booster as prior art, thereby further improving the 
response of the hydraulic brake system. Since the switching valve 64 is 
controlled by the fluid pressure of the ACC 46, the fluid pressure in the 
power chamber of the hydraulic booster never produces loss, thereby 
further improving the response of the hydraulic brake system. 
Moreover, when the fluid pressure of the ACC 46 drops, the MCY pressure of 
the MCY 3 is supplied to the WCYs 58, 59 through the switching valve 64. 
Since the MCY pressure never functions as pilot pressure of the switching 
valve 64 for controlling the switching, the interruption between the fluid 
chamber 57 of the MCY 3 and the WCYs 58, 59 is securely prevented when the 
fluid pressure drops. In addition, the switching of the switching valve 64 
is not influenced by the MCY pressure when the fluid pressure drops, 
thereby further improving the response and facilitating the setting of the 
receiving area receiving the pilot pressure of the switching valve 64 and 
the spring force of the spring 67. 
Further, in both cases where the fluid pressure is normal and where the 
fluid pressure drops, the volume of the fluid chamber 57 of the MCY 3 is 
never increased by the switching of the switching valve 64, thereby 
securely preventing the pedal stroke during braking from being increased. 
Furthermore, the piston 70 of the switching valve 64 moves only when the 
fluid pressure of the ACC 46 drops while the piston 70 is independent of 
the operation of the hydraulic booster 2 so as not to move when the fluid 
pressure is normal, thereby improving the durability of the O-rings 71, 72 
of the piston 70. In addition, in this case, the case where the fluid 
pressure drops is quite rare and in most cases, the fluid pressure is 
normal, thereby further improving the durability of the O-rings 71, 72. 
Though the present invention is applied to the hydraulic booster system 
using the brake pressure-producing device such that the control valve is 
built in the power piston in the above embodiment, the present invention 
can be applied to a booster system using a brake pressure-producing device 
being of such a type that a control valve for the hydraulic booster is 
disposed out of a power piston as disclosed in the aforementioned 
publication. 
Though the pressurized fluid in the power chamber 30 is supplied to the 
WCYs 58, 59 relating to the other circuit through the annular chamber 51 
disposed in the hydraulic booster 2 and communicating with the power 
chamber 30 in the above embodiment, the pressurized fluid in the power 
chamber 30 can be supplied directly to the WCYs 58, 59 relating to the 
other circuit without the annular chamber. 
As apparent from the above description, in the hydraulic brake system 
according to the present invention, since the power chamber is 
communicated with the brake cylinders by the switching valve whenever the 
fluid pressure in the fluid pressure source exceeds the predetermined 
pressure, the fluid pressure introduced in the power chamber can be 
introduced directly to the brake cylinders when the braking operation is 
performed, thereby rapidly actuating the brakes. As a result of this, the 
response of the hydraulic brake system can be improved. Because the 
switching valve is controlled by the fluid pressure in the fluid pressure 
source, the fluid pressure, to be introduced to the brake cylinders, in 
the power chamber of the hydraulic booster never produces loss, thereby 
further improving the response of the hydraulic brake system. 
Since the fluid chamber of the master cylinder is communicated with the 
brake cylinders through the switching valve when the fluid pressure in the 
fluid pressure source is less than the predetermined pressure, the brakes 
are securely actuated during the braking operation even when the fluid 
pressure in the fluid pressure source becomes less than the predetermined 
pressure. 
Also according to the present invention, the switching of the switching 
control piston valve is not influenced by the master cylinder pressure 
during the fluid pressure drops, thereby further improving the response 
and facilitating the setting of the receiving area receiving the pilot 
pressure of the switching control piston valve and the biasing force of 
biasing means such as a spring for biasing the switching control piston 
valve. 
In addition, since the master cylinder pressure is never exerted as pilot 
pressure for controlling the switching on the switching control piston 
valve when the fluid pressure drops, the interruption between the fluid 
chamber of the master cylinder and the brake cylinders is securely 
prevented during the fluid pressure drops. 
Further, in both cases where the fluid pressure is normal and where the 
fluid pressure drops, the volume of the fluid chamber of the master 
cylinder is never increased by the switching of the switching control 
piston valve, thereby securely preventing the pedal stroke during braking 
from being increased. 
Furthermore, the switching control piston valve is moved only when the 
fluid pressure drops while the switching control piston valve is 
independent of the operation of the hydraulic booster so as not to move 
when the fluid pressure is normal, thereby improving the durability of the 
seals of the switching control piston valve. In addition, in this case, 
the case where the fluid pressure drops is quite rare and in most cases, 
the fluid pressure is normal, thereby further improving the durability of 
the seals.