Hydraulic braking system for an automotive vehicle

A hydraulic braking system for an automotive vehicle having a power source for generating a hydraulic power pressure, a reservoir, a master cylinder having a master piston, a hydraulic booster for actuating the master cylinder in response to depression of a brake pedal and a plurality of wheel brake cylinders connected to the master cylinder for braking respective road wheels. The hydraulic booster is provided with a power piston which is larger in diameter than the master piston. The power piston transmits a force to the master piston through a closed chamber which is defined between the power piston and the master piston, when the hydraulic power pressure is supplied to the hydraulic booster in response to depression of the brake pedal. The closed chamber is filled with a brake fluid which is supplied from the reservoir. Thus, a small stroke of the power piston will suffice for operating the master piston at a predetermined stroke, so that the stroke of the brake pedal is reduced. Also, a braking force is ensured with the power piston mechanically connected to the master piston, when the hydraulic booster, for example, does not operate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a hydraulic braking system for use in an 
automotive vehicle, and more particularly to a hydraulic braking system 
which comprises a hydraulic booster for actuating a master cylinder in 
response to operation of a brake pedal with a hydraulic power pressure 
supplied from a power source. 
2. Description of the Prior Art 
In conventional service braking systems for an automotive vehicle, there 
are provided a plurality of hydraulic circuits connecting wheel brake 
cylinders mounted on road wheels with a hydraulic braking pressure 
generator such as a master cylinder, so that when one of the hydraulic 
circuits is broken, braking operation is performed by the rest of the 
hydraulic circuits. In general, a tandem master cylinder is used in a 
conventional dual circuits system. 
In order to reduce a depressing force applied on a brake pedal in braking 
operation, the hydraulic braking system is provided with a servo unit 
which is referred as a servo or a booster and which utilizes compressed 
air, intake manifold vacuum (for a vacuum booster), or hydraulic pressure 
(for a hydraulic booster) as a power source. The hydraulic booster is a 
booster which actuates the hydraulic braking pressure generator such as 
the master cylinder by the hydraulic power pressure supplied from the 
power source in response to depression of the brake pedal. In Japanese 
Patent Laid-open Publication No. 59-209948, for example, disclosed is a 
hydraulic braking system in which the hydraulic booster is associated with 
a tandem master cylinder and which operates as a conventional tandem 
master cylinder when the hydraulic booster does not operate. 
Further, It has been proposed that a hydraulic pressure generated by the 
hydraulic booster in response to depression of the brake pedal is applied 
directly to one of the hydraulic circuits. For example, as shown in 
Japanese Patent Laid-open Publication No. 59-227552, the hydraulic 
pressure generated by the hydraulic booster is applied to rear wheel brake 
cylinders in a front-rear dual circuits system in order to reduce the 
stroke of the brake pedal. 
As for the above-described conventional hydraulic braking system, in the 
case where the hydraulic booster becomes insufficient to fulfill its boost 
function, or the case where the hydraulic power pressure disappears due to 
stoppage of the power source or other defects so that the hydraulic 
booster becomes incapable of obtaining any boost function, the braking 
force on the front road wheels is ensured by the master cylinder, but a 
large depressing force shall be applied on the brake pedal in order to 
obtain the necessary braking force. 
In the Japanese Patent Laid-open Publication No. 62-149547, there is 
disclosed a system, in which a booster or an auxiliary cylinder is 
arranged in parallel with a hydraulic braking pressure generator and a 
control valve is provided for applying to wheel brake cylinders a 
hydraulic braking pressure from a master cylinder increased by the 
auxiliary cylinder when a hydraulic pressure generated in a hydraulic 
pressure chamber of a hydraulic booster or a power source is less than 
that in the master cylinder by a predetermined value or above, while 
applying the hydraulic braking pressure from the master cylinder to the 
wheel brake cylinders without increasing the hydraulic braking pressure in 
the case other than the above. Thereby, even if the boost function of the 
hydraulic booster cannot be obtained, the hydraulic braking pressure from 
the master cylinder is increased by the auxiliary cylinder to be applied 
to the wheel brake cylinders, so that a large depressing force does not 
have to be applied on the brake pedal. 
However, in the above described hydraulic braking system, there must be 
provided an auxiliary cylinder or the like in order to ensure a braking 
force when a source for reducing the stroke of the brake pedal, such as 
the hydraulic booster does not operate. As for the auxiliary cylinder, 
various embodiments thereof are disclosed in the above publication. 
However, any of those auxiliary cylinders must be incorporated into the 
conventional hydraulic braking system having the tandem master cylinder 
and the hydraulic booster for example, integrally or separately, so that 
the braking system as a whole tends to be expensive, large and heavy. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a hydraulic 
braking system for an automotive vehicle having a hydraulic booster and a 
master cylinder, which reduces a stroke of a brake pedal and which ensures 
a braking force when a source for reducing the stroke does not operate. 
It is another object of the present invention to provide a hydraulic 
braking system for an automotive vehicle which reduces the stroke of the 
brake pedal, and which is relatively small in size, light in weight and 
economical to manufacture. 
In accomplishing these and other objects, a hydraulic braking system for an 
automotive vehicle according to the present invention comprises a power 
source for generating a hydraulic power pressure, a reservoir for storing 
an amount of brake fluid, a master cylinder which has a housing defining 
therein a first bore and a master piston slidably disposed in the first 
bore, and which introduces the brake fluid into the first bore from the 
reservoir and generates a hydraulic braking pressure in response to 
depression of a brake pedal, and a hydraulic booster which has a housing 
defining therein a boost chamber and a second bore communicated therewith 
and aligned coaxially with the first bore. The hydraulic booster has a 
power piston which is slidably disposed in the second bore and actuates 
the master piston with the hydraulic power pressure supplied into the 
boost chamber from the power source in response to depression of the brake 
pedal. The power piston is larger in diameter than the master piston and 
transmits a force to the master piston through a closed chamber which is 
defined between the power piston and the master piston, and which is 
filled with the brake fluid supplied from the reservoir. The hydraulic 
braking system further comprises a plurality of wheel brake cylinders 
which is connected to the master cylinder and brakes respective road 
wheels. 
The above-described hydraulic braking system may further comprise first 
one-way valve means which is connected to the closed chamber and which 
permits the flow of the brake fluid from the reservoir to the closed 
chamber in response to a difference in hydraulic pressure therebetween. 
The above-described braking system may further comprise second one-way 
valve means which is connected to the closed chamber and which permits the 
flow of the brake fluid from the closed chamber to the boost chamber in 
response to a difference in hydraulic pressure therebetween. 
In the above-described braking system, one end of the master piston may be 
slidably disposed in the first bore and the other end of the master piston 
is received in the second bore which is divided into the closed chamber 
and a fluid chamber communicating with the reservoir by a first annular 
cup seal which is disposed in the second bore for encircling a periphery 
of the master piston. The first annular cup seal permits the flow of the 
brake fluid from the fluid chamber to the closed chamber in response to a 
difference in hydraulic pressure therebetween and blocks the counterflow 
thereof. Also, a second annular cup seal may be disposed in the second 
bore for encircling the power piston. The second annular cup seal permits 
the flow of the brake fluid from the closed chamber to the boost chamber 
in response to a difference in hydraulic pressure therebetween and blocks 
the counterflow thereof. A spring is disposed between the power piston and 
the master piston, and the spring biases the pistons away from each other.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, there is illustrated a hydraulic braking system of an 
embodiment of the present invention, which includes a hydraulic pressure 
generator (hereinafter referred to simply as a pressure generator) 1 
having a tandem master cylinder 10 and a hydraulic booster 20. A power 
source 40 and a reservoir 41 are connected to the pressure generator 1. A 
depressing force applied on a brake pedal 2 is transmitted as a braking 
force to the pressure generator 1. In response to this braking force, a 
hydraulic pressure is generated by the pressure generator 1 and applied to 
wheel brake cylinders 51a to 54a mounted on front road wheels 51, 52 and 
rear road wheels 53, 54, through a first hydraulic circuit 71 and a second 
hydraulic circuit 72 respectively. 
A housing of the pressure generator 1 comprises a housing 1a having a 
cylinder bore 10a formed therein and a housing 1b having a cylinder bore 
20a formed therein. Both cylinder bores 10a, 20a are joined together 
coaxially and communicated with each other. The cylinder bore 20a is 
larger in diameter than the cylinder bore 10a and has a stepped bore 
formed in the vicinity of a joint where both cylinder bores 10a, 20a are 
joined together. The stepped bore has a large-diameter portion, into which 
an annular member 10c is fitted. The annular member 10c is held between 
the stepped portion of the cylinder bore 20a and an open end portion of 
the housing 1a. The inner diameter of the annular member 10c is 
substantially equal with that of the cylinder bore 10a. The annular member 
10c has an inner surface formed with an annular recess, in which a cup 
seal 4a is received. Further, the cylinder bore 20a has an inner surface 
formed with an annular recess, in which a cup seal 4b is received. 
A tandem master cylinder 10 is provided with a first piston 11 and a second 
piston 15. The first piston 11 has a cylindrical body portion which is 
fitted into the annular member 10c, and a land portion which is formed on 
one end of the first piston 11 and which is fluid-tightly and slidably 
received in the cylinder bore 10a. The second piston 15 is disposed 
adjacent to the first piston 11 and fluid-tightly and slidably received in 
the cylinder bore 10a. 
The cup seal 4a is interposed between the annular member 10c and the body 
portion of the first piston 11 thereby to hold the first piston 11 
fluid-tightly and slidably. Further, a first fluid chamber 13 is defined 
between the annular member 10c having the cup seal 4a and the land portion 
of the first piston 11. The first fluid chamber 13 communicates with a 
reservoir 41 through a port 13a. Further, in the cylinder bore 10a, a 
first pressure chamber 12 is defined between the land portion of the first 
piston 11 and the second piston 15. The first pressure chamber 12 
communicates with a first hydraulic circuit 71 through a port 12a. 
The first piston 11 has a hole 11a extending axially from its one end 
toward its center and a hole 11c formed radially. Both holes 11a, 11c 
communicate with each other through a hole 11d formed axially in the first 
piston 11. Further, a hole 11e is formed axially in a peripheral edge 
portion of the first piston 11, and covered with a cup seal 11f at its end 
open to the first pressure chamber 12, whereby a check valve is formed. A 
valve member 14a mounted on one end of a valve rod 14 is slidably received 
in the hole 11a of the first piston 11 so as to face with the hole 11d. 
The valve member 14 is restricted from moving toward the second piston 15 
by a retainer 14c. A large-diameter portion formed on the other end of the 
valve rod 14 is slidably received in a hole 15b formed in the second 
piston 15, and restricted from moving toward the first piston 11 by a 
retainer 14b. A return spring 14d is mounted between the retainers 14b, 
14c so as to bias the first piston 11 away from the second piston 15, so 
that the opposite ends of the valve rod 14 are engaged with the retainers 
14b, 14c in the normal state. 
Accordingly, the valve member 14a is positioned away from the hole 11d, and 
a brake fluid supplied from the reservoir 41 to the first fluid chamber 13 
through the port 13a is then supplied to the first pressure chamber 12 
through the hole 11e, and also through the hole 11c, the hole 11d and the 
hole 11a of the first piston 11 to be filled within the first pressure 
chamber 12. As a result, when the first piston 11 is pressed toward the 
second piston 15 to be moved against the biasing force of the return 
spring 14d, the hole 11e is closed by the cup seal 11f, and the hole 11d 
is closed by the valve member 14a, so that the first pressure chamber 12 
is held in the closed state except the port 12a thereby to increase the 
pressure of the brake fluid in the first pressure chamber 12 in response 
to the sliding motion of the first piston 11. 
The second piston 15 is disposed between the closed end 10b of the cylinder 
bore 10a and the first piston 11, and fluid-tightly and slidably received 
in the cylinder bore 10a. The second piston 15 has opposite ends formed 
with a land portion of the same diameter respectively. A second fluid 
chamber 17 is defined between both land portions of the second piston 15, 
and a second pressure chamber 16 is defined between the second piston 15 
and the closed end 10b of the cylinder bore 10a. The second pressure 
chamber 16 communicates with a second hydraulic circuit 72 through a port 
16a, and the second fluid chamber 17 communicates with the reservoir 41 
through a port 17a. 
The second piston 15 has a pair of holes 15a, 15b formed axially and a hole 
15c formed radially. Both holes 15a, 15c communicate with each other 
through a hole 15d formed axially in the second piston 15. Further, a hole 
15e is formed axially in a peripheral edge portion of the second piston 
15, and covered with a cup seal 15f at its end open to the second pressure 
chamber 16. A valve member 18a mounted on one end of a valve rod 18 is 
slidably received in the hole 15a of the second piston 15 so as to face 
with the hole 15d, and restricted from moving toward the closed end 10b by 
a retainer 18c. A large-diameter portion formed on the other end of the 
valve rod 18 is slidably received in a retainer 18b, and restricted from 
moving toward the second piston 15. A return spring 18d is mounted between 
the retainers 18b, 18c, so that the second piston 15 is biased to be away 
from the closed end 10b, while the second piston 15 is restricted from 
moving toward the first piston 11 by a bolt 19. Accordingly, in the normal 
state, the valve member 18a is positioned away from the hole 15d, and the 
brake fluid is supplied from the reservoir 41 to the second fluid chamber 
17 and the second pressure chamber 16 through the port 17a to be filled 
within the chambers. 
Accordingly, when the first piston 11 slides toward the second piston 15, 
the first pressure chamber 12 is contracted to increase the pressure of 
the brake fluid in the first pressure chamber 12, while the second piston 
15 slides against the biasing force of the return spring 18d, whereby the 
hole 15d is closed by the valve member 18a to increase the pressure of the 
brake fluid in the second pressure chamber 16. 
In the housing 1b joined with the housing 1a, a boost chamber 20b of the 
hydraulic booster 10 is defined. The boost chamber 20b communicates with 
the cylinder bore 20a. A power piston 5 is fluid-tightly and slidably 
received in the cylinder bore 20a with the cup seal 4b disposed 
therebetween. The power piston 5 has a recess 5a which is formed at its 
end facing the first piston 11, a bore 5b which is formed axially in the 
center and communicates with the recess 5a, and a bore 5e which is larger 
in diameter than the bore 5b. A reaction piston 22 is fluid-tightly and 
slidably received in the bore 5b. 
Accordingly, a closed chamber 4 is defined around the body portion of the 
first piston 11 located between the cup seal 4b, into which the power 
piston 5 is fitted, and the cup seal 4a, into which the first piston 11 is 
fitted. As shown in FIG. 1, a lip portion of each of the cup seals 4a, 4b 
has the tip directed toward the boost chamber 20b. Namely, the cup seal 4a 
constitutes one-way valve means for permitting the flow of the brake fluid 
from the first fluid chamber 13 to the closed chamber 4, while blocking 
the flow of the brake fluid from the closed chamber 4 to the first fluid 
chamber 13, in accordance with a difference in hydraulic pressure between 
the first fluid chamber 13 and the closed chamber 4. Further, the cup seal 
4b constitutes one-way valve means for permitting the flow of the brake 
fluid from the closed chamber 4 to the boost chamber 20b, while blocking 
the flow of the brake fluid from the boost chamber 20b to the closed 
chamber 4, in accordance with a difference in hydraulic pressure between 
the closed chamber 4 and the boost chamber 20b. The power piston 5 which 
is fitted into the cylinder bore 20a is larger in diameter than the first 
piston 11 which is fitted into the cylinder bore 10a, so that the 
pressure-applied area of the power piston in the closed chamber 4 is 
larger than that of the first piston 11. 
In the hydraulic booster 20, the power piston 5 is provided with a retainer 
(not shown) at its end extending toward the brake pedal 2, and normally 
biased toward the brake pedal 2 by a spring 7 mounted between the retainer 
and the housing 1b. The power piston 5 has in the middle thereof a 
shoulder portion which abuts on the housing 1b to restrict the power 
piston 5 from sliding toward the brake pedal 2. In the recess 5a formed on 
the power piston 5 at its end facing the first piston 11, a spring 6 is 
disposed with one end thereof seated on the bottom of the recess 5a, while 
the other end of the spring 6 is seated on a recess formed on one end 
surface of the first piston 11, so that the power piston 5 and the first 
piston 11 are biased away from each other. 
In the reaction piston 22, there is formed an elongated hole 22a extending 
coaxially with the axis of the reaction piston 22, and a through-hole 22b 
extending perpendicularly to the elongated hole 22a. A pin 5h which is 
fixed to the power piston 5 is disposed in the elongated hole 22a, so that 
the reaction piston 22 is restricted from sliding at least toward the 
brake pedal 2 with respect to the power piston 5. One end of an input rod 
3 is connected to the brake pedal 2, and the other end of the input rod 3 
is provided with a spherical head which is inserted in the bore 5e of the 
power piston 5 and received in a recess formed at an end portion of the 
reaction piston 22, and which is engaged with the projection formed on the 
inner surface of the recess. In the power piston 5, there is formed 
radially a through-hole 5f which is aligned with the through-hole 22b when 
the reaction piston 22 is positioned most closely to the brake pedal 2 and 
which is larger in diameter than the through-hole 22b. 
A support lever 24 is pivotally connected at its one end to the housing 1b 
by a pin 1c for pivotal movement within the boost chamber 20b, and a 
spherical head of the support lever 24 is fitted into the through-hole 22b 
of the reaction piston 22. And, one head of a control lever 25, which is 
pivotally connected to the support lever 24 by a pin 24a, is fitted into 
the through-hole 5f of the power piston 5. In the other head of the 
control lever 25, there is defined a hole around the pin 1c. Accordingly, 
when the reaction piston 22 slides toward the output rod 14 with respect 
to the power piston 5 which is urged toward the brake pedal 2, a rotating 
force is exerted on the support lever 24 so as to pivotally move the 
support lever 24 clockwise about the pin 1c. At that time, since one head 
of the control lever 25 is retained in the through-hole 5f of the power 
piston 5, the other head of the control lever 25 is rotated 
counterclockwise about the pin 24a and hence moved in the sliding 
direction of the reaction piston 22. As a result, the other head of the 
control lever 25 is displaced in response to movement of the reaction 
piston 22 until it comes into contact with the bottom of the bore 5b. 
The housing 1b has a spool-valve bore extending substantially in parallel 
with the power piston 5 and communicating with the boost chamber 20b, and 
a spool valve 28 is fitted into the spool-valve bore. The spool valve 28 
has a spool 26 which is slidably received in a spool bore 27a formed in a 
cylinder 27 substantially in parallel with the power piston 5. One end of 
the spool bore 27a is fluid-tightly plugged by a closure member 27f. In 
the spool 26, there is formed axially a hole 26a, and formed radially a 
hole 26b communicating with the hole 26a. One end of the spool 26 is 
positioned in the boost chamber 20b and connected to one end of a control 
rod 29. The other end of the control rod 29 is slidably mounted on a 
recess formed in the housing 1b, and the head of the control lever 25 is 
fitted into a through-hole 29a radially bored in the control rod 29. 
Between the cylinder 27 and a retainer 29b supported at one end of the 
control rod 29, a spring 29c is mounted so as to normally bias the spool 
26 toward the pin 1c. The hole 26a normally opens to the boost chamber 20b 
at the junction of the spool 26 and the control rod 29. When the control 
lever 25 is in its initial position, the hole 26a of the spool 26 
communicates with the reservoir 41 through a hole 27b radially bored in 
the cylinder 27. Thus, the boost chamber 20b also communicates with the 
reservoir 41 and is filled with the brake fluid under the atmospheric 
pressure. 
A hole 27c communicating with the power source 40 is formed in the cylinder 
27 with a predetermined distance from the hole 27b toward the control rod 
29. The hole 27c is normally closed by the peripheral surface of the spool 
26. Between the hole 27c and the one end of the spool 26 facing the 
control rod 29, an annular groove 27e is formed on the inner surface of 
the cylinder 27, and an annular groove 26c is formed on the peripheral 
outer surface of the spool 26 in opposing relation to the annular groove 
27e. When the spool 26 is moved toward the closure member 27f in response 
to movement of the control lever 25, the hole 27b of the cylinder 27 is 
closed. The hole 27c in turn faces the annular groove 26c of the spool 26, 
and the annular groove 27e faces the hole 26b. Consequently, the hole 27c 
communicates with the hole 26a. 
Accordingly, the hydraulic power pressure of the power source 40 is 
introduced into the boost chamber 20b to increase the hydraulic pressure 
therein, so that the reaction force is thereby transmitted to the brake 
pedal 2 through the reaction piston 22, and simultaneously the increased 
hydraulic pressure is applied to the power piston 5, so that the power 
piston 5 is forced to move toward the first piston 11. The power piston 5 
moves until the pin 5h comes into contact with an end portion of the 
elongated hole 22a at maximum with respect to the reaction piston 22. 
Thereby, the relative position of the control lever 25 and the support 
lever 24 becomes that in its initial state. Accordingly, the control lever 
25 is moved clockwise to retract the control rod 29 toward the brake pedal 
2. Thereby, the hole 27c of the cylinder 27 is closed, and in turn the 
hole 27b communicates with the hole 26a, of the spool 26 to lower the 
hydraulic pressure in the boost chamber 20b so that the power piston 5 is 
moved toward the brake pedal 2. With this operation performed repeatedly, 
the hydraulic power pressure within the boost chamber 20b is regulated so 
as to boost the master cylinder 10. 
The first pressure chamber 12 of the tandem master cylinder 10 communicates 
with one circuit of the dual circuits, that is, it communicates with the 
wheel brake cylinders 51a, 52a of the front road wheels 51, 52 through the 
first hydraulic circuit 71 in the present embodiment, while the second 
pressure chamber 16 of the tandem master cylinder 10 communicates with the 
other circuit, that is, it communicates with the wheel brake cylinders 
53a, 54a of the rear road wheels 53, 54 through the second hydraulic 
circuit 72. The above-described dual circuits may be arranged reversely in 
the front and rear road wheels, or may be arranged in so-called diagonal 
circuit system. 
The power source 40 comprises a fluid pump 43 driven by an electric motor 
42 and is so structured that its input side is connected to the reservoir 
41 while its output side is connected t an accumulator 44 via a check 
valve 45, and the hydraulic power pressure is supplied to necessary 
portions via the accumulator 44. Further, the electric motor 42 is 
intermittently controlled by a controller (not shown) in response to a 
signal of a pressure sensor 40a, so that the hydraulic power pressure is 
maintained to be at a predetermined value. 
The operation of the above described embodiment will now be explained. In 
the case where the brake pedal 2 is not depressed as shown in FIG. 1, the 
first pressure chamber 12 and the first fluid chamber 13 of the tandem 
master cylinder 10 communicate with each other, and communicate with the 
wheel brake cylinders 51a, 52a of the front road wheels 51, 52 and the 
reservoir 41 respectively, so that the brake fluid filled in each of these 
chambers is under a pressure equal to the pressure in the reservoir 41, 
that is, substantially under the atmospheric pressure. Further, the spring 
6 is expanded, and the closed chamber 4 is filled with the brake fluid. 
Accordingly, the power piston 5 is fluidly connected to the first piston 
11 through the brake fluid filled in the closed chamber 4. 
On the other hand, when the power source 40 is actuated, the hydraulic 
power pressure is supplied to the hole 27c of the hydraulic booster 20, 
whereas the hydraulic booster 20 does not operate since the hole 27c is 
closed. The brake fluid filled in each of the second pressure chamber 16 
and the second fluid chamber 17 communicates with the reservoir 41 through 
the port 17a, and it is substantially under the atmospheric pressure, so 
that the wheel brake cylinders 53a, 54a communicating with the second 
pressure chamber 16 through the port 16a and the second hydraulic circuit 
72 are also under the atmospheric pressure. 
In the case where the depressing force is applied on the brake pedal 2, the 
reaction piston 22 is pushed with the input rod 3. And, when the reaction 
piston 2 is moved until it comes into contact with the bottom of the bore 
5b of the power piston 5, the control lever 25 is rotated counterclockwise 
with respect to the support lever 24, so that the head of the control 
lever 25 pushes the spool 26. Thereby, the hydraulic power pressure is 
introduced from the power source 40 into the boost chamber 20b, so that 
the boost force is applied to the first piston 11 with the power piston 5, 
and the reaction force is transmitted to the brake pedal 2 with the 
reaction piston 22. 
When the first piston 11 starts to slide with the power piston 5, the hole 
11d is closed by the valve member 14a, and the hydraulic braking pressure 
is supplied to the wheel brake cylinders 51a, 52a in response to the 
contraction of the first pressure chamber 12. At the same time, the second 
piston 15 slides, and the hydraulic braking pressure is supplied to the 
wheel brake cylinders 53a, 54a in response to the contraction of the 
second pressure chamber 16. 
In the above-described braking operation, since the hydraulic pressure in 
the closed chamber 4 is less than that in the boost chamber 20b, and 
higher than that in the first fluid chamber 13, the closed chamber 4 
maintains the quantity of the brake fluid filled therein under the 
condition that the spring 6 is expanded, that is, under the condition that 
the power piston 5 is away from the first piston 11. Thus, the boost force 
is transmitted from the power piston 5 to the first piston 11 through the 
fluid, or the brake fluid filled in the closed chamber 4. Since the power 
piston 5 is larger in diameter than the first piston 11 and has the 
pressure applied area larger than that of the first piston 11, the stroke 
of the first piston 11 comes to be larger than that of the power piston 5 
in inverse proportion to the ratio of the pressure applied area of the 
power piston 5 to that of the first piston 11. In other words, a small 
stroke will suffice for the stroke of the power piston 5 necessary for 
operating the first piston 11 at the predetermined stroke, that is, the 
stroke of the brake pedal 2. Accordingly, in the above described braking 
operation, the stroke of the brake pedal 2 is reduced, comparing with that 
of the prior art system. 
In the case where the hydraulic power pressure disappears due to some 
defects of the power source 40, or the hydraulic pressure in the boost 
chamber 20b becomes less than that in the closed chamber 4 due to some 
defects of the hydraulic booster 20 itself for example, when the power 
piston 5 is moved by depressing the brake pedal 2, the brake fluid in the 
closed chamber 4 flows into the boost chamber 20b. Thereby, the end 
surface of the power piston 5 comes into contact with the end surface of 
the first piston 11 against the biasing force of spring 6, so that the 
power piston 5 is mechanically connected to the first piston 11. Thus, a 
sufficient braking force is ensured even if the boost force of the 
hydraulic booster 20 disappears. 
When the brake pedal 2 is released, the hydraulic pressure in the closed 
chamber 4 is decreased, and the brake fluid is supplied from the first 
fluid chamber 13 to the closed chamber 4 due to a difference in hydraulic 
pressure between the first fluid chamber 13 and the closed chamber 4, so 
that the brake fluid is filled in the closed chamber 4. Consequently, soon 
after the boost force of the hydraulic booster 20 becomes sufficient and 
the boost function is recovered, the function of reducing the stroke is 
obtained. 
As described above, according to the present embodiment, the stroke of the 
brake pedal 2 may be reduced with a simple structure made within the range 
of design of the existing hydraulic braking system, or with the prior 
hydraulic braking system modified in such a manner that the first piston 
11 is axially extended and the closed chamber 4 is defined by the cup 
seals 4a, 4b different from each other in diameter. In addition, when the 
boost force of the hydraulic booster 20 disappears, the tandem master 
cylinder 10 is mechanically operated by the brake pedal 2 to ensure the 
braking force. 
FIG. 2 shows a part of the hydraulic braking system according to another 
embodiment of the present invention. The remaining structure of this 
embodiment is substantially same as that shown in FIG. 1. In this 
embodiment, check valves 4c, 4d are additionally provided for one-way 
valve means, in lieu of the cup seals 4a, 4b shown in FIG. 1. Therefore, 
seal members contacted with the power piston 5 and the first piston 11 for 
defining the closed chamber 4 are not required to have the one-way valve 
function, so that the seal members may be selected on the basis of only 
sealing performance. Further, the check valves 4a, 4b may be of a 
small-sized type, so that they may be easily disposed in a surplus space. 
It should be apparent to one skilled in the art that the above-described 
embodiments are merely illustrative of but a few of the many possible 
specific embodiments of the present invention. Numerous and various other 
arrangements can be readily devised by those skilled in the art without 
departing from the spirit and scope of the invention as defined in the 
following claims.