Back-up pressure intensifier for master cylinder failure

A braking unit includes an intensifier mechanism for intensifying a braking liquid pressure of a master cylinder upon failure of a liquid pressure booster. The braking unit comprises the intensifier mechanism for intensifying the braking liquid pressure of the master cylinder when actuated, and a control valve which makes the intensifier mechanism operable upon failure of the liquid pressure booster. The control valve comprises a valve member which is urged by a spring tensioned with a given load and which is urged in the opposite direction from the spring by a liquid pressure of a power chamber of the liquid pressure booster. The valve member makes the intensifier mechanism operable whenever the resilience of the spring exceeds the urging force produced by the liquid pressure in the power chamber of the liquid pressure booster. This prevents an operational lag of the intensifier mechanism in the event the liquid pressure booster fails.

FIELD OF THE INVENTION 
The invention relates to a braking unit, and more particularly, to a 
braking unit including an intensifier mechanism which intensifies a 
braking liquid pressure in a master cylinder upon failure or sinking of a 
liquid pressure booster. 
DESCRIPTION OF THE PRIOR ART 
A braking unit is known in the art including a master cylinder which 
applies a braking liquid pressure to a wheel cylinder, a liquid pressure 
booster disposed between a brake pedal and the master cylinder for 
boosting a force with which the brake pedal is depressed at a given ratio 
before it is transmitted to the master cylinder, an intensifier mechanism 
disposed in a braking liquid path which provides a communication between 
the wheel cylinder and the master cylinder and adapted to be actuated to 
intensify the braking liquid pressure in the master cylinder for 
transmission to the wheel cylinder, and a control valve for actuating the 
intensifier mechanism whenever the liquid pressure booster has failed or 
has sunk. (See Japanese Patent Publication No. 29,011/1994). 
In a braking unit of the art as described above, a comparison is made 
between the liquid pressure in a power chamber of the liquid pressure 
booster and the liquid pressure in the master cylinder. The control valve 
is operated to switch a flow path for the braking liquid to actuate the 
intensifier mechanism when the liquid pressure in the power chamber falls 
below the liquid pressure in the master cylinder. This allows the braking 
liquid pressure in a master cylinder to be intensified by a factor on the 
order of two by the intensifier mechanism, and intensified braking liquid 
pressure is transmitted to the wheel cylinder. This assures a reliable 
brake operation if the liquid pressure booster should lose its booster 
function. Thus the intensifier mechanism functions as a fail-safe unit. 
However, in a conventional arrangement, if the liquid pressure in the power 
chamber of the liquid pressure booster does not reduce to zero upon 
failure thereof, the control valve is only operable to actuate the 
intensifier mechanism after the depression of the brake pedal is continued 
and the liquid pressure in the master cylinder rises above the liquid 
pressure in the power chamber. Accordingly, in such conventional 
arrangement, the actuation of the intensifier mechanism tends to lag if 
any liquid pressure remains in the power chamber when the liquid pressure 
booster has failed, which is a disadvantage. 
SUMMARY OF THE INVENTION 
The invention relates to a braking unit including a master cylinder for 
applying a braking liquid pressure to a wheel cylinder, a liquid pressure 
booster disposed between a brake pedal and the master cylinder for 
boosting a force with which the brake pedal is depressed at a given ratio 
before it is transmitted to the master cylinder, an intensifier mechanism 
disposed in a braking liquid path which provides a communication between 
the wheel cylinder and the master cylinder that is adapted to be actuated 
to intensify the braking liquid pressure in the master cylinder for 
transmission to the wheel cylinder, and a control valve for actuating the 
intensifier mechanism upon failure of the liquid pressure booster. In view 
of the foregoing, in accordance with the invention, the control valve 
includes a valve member which is urged to an operative position by a 
spring having a preset load to make the intensifier mechanism operable and 
is moved to its inoperative position under the influence of a liquid 
pressure in a power chamber of the liquid pressure booster or a liquid 
pressure of a source of liquid pressure associated with the liquid 
pressure booster which acts in the opposite direction from the direction 
in which the resilience of the spring acts to make the intensifier 
mechanism inoperable. The valve member is moved from its inoperative to 
its operative position whenever the force which it experiences under the 
influence of the liquid pressure in the power chamber or the liquid 
pressure in the source of liquid pressure associated with the liquid 
pressure booster is reduced below the urging force from the spring. 
With this arrangement, when the liquid pressure booster or a source of 
liquid pressure associated therewith fails, and the liquid pressure in the 
power chamber of the liquid pressure booster or the liquid pressure in the 
source associated therewith provides an urging force which is less than 
the resilience of the spring, the control valve becomes operative to make 
the intensifier mechanism operable. In this manner, the intensifier 
mechanism is immediately actuated as soon as the liquid pressure in the 
master cylinder is generated, thus effectively preventing any operational 
lag in the actuation of the intensifier mechanism when the liquid pressure 
booster or a source of liquid pressure associated therewith fails. 
Above and other objects, features and advantages of the invention will 
become apparent from the following description of several embodiments 
thereof with reference to the drawings.

DETAILED DESCRIPTION OF EMBODIMENTS 
Referring to the drawings, several embodiments of the invention will now be 
described. Initially referring to FIG. 1, a brake pedal 1 is connected to 
an input shaft of a liquid pressure booster 2, which is constructed in a 
known manner. The booster 2 includes an output shaft which is connected to 
a piston of a master cylinder 3. When the brake pedal 1 is depressed, the 
booster 2 boosts the force with which the brake pedal 1 is depressed at a 
given ratio before it is transmitted to the piston of the master cylinder 
3. 
In the present embodiment, the master cylinder 3 is of a tandem type, and 
operates to transmit a braking liquid pressure which is generated in 
response to the depression of the brake pedal 1 to front wheel cylinders 5 
through a braking liquid path 4, and simultaneously transmit such pressure 
to rear wheel cylinders, not shown, through another braking liquid path 6. 
An intensifier mechanism 7 and a control valve 8, to be described later, 
are disposed in the braking liquid path 4. Similarly, an intensifier 
mechanism 7 and a control valve 8 are disposed in the path 6 associated 
with the rear wheel cylinders, but they are not shown and will not be 
described herein since they are identical to those disposed in the path 4. 
When the booster mechanism 7 is actuated by the control valve 8, the 
braking liquid pressure which is generated in the master cylinder 3 is 
boosted before it is transmitted to the front wheel cylinders 5. 
The liquid pressure booster 2 is connected through a liquid path 11 to an 
accumulator 12, and also connected to a pump 13 which is operated by a 
motor 18. A braking liquid which is contained in a reservoir 14 is pumped 
by the pump 13 to the accumulator 12, and thence to a power chamber of the 
liquid pressure booster 2. The braking liquid which is displaced from the 
booster 2 is returned to the reservoir 14 through a liquid path 15. It 
will be seen that a check valve 17 is disposed in a path extending between 
the reservoir and the pump and maintains a pump discharge pressure. It is 
to be noted that in the present embodiment, the liquid pressure from the 
accumulator 12 also acts upon the intensifier mechanism 7 through a 
braking liquid path 16. 
Referrring to FIG. 2, the intensifier mechanism 7 includes a casing 21 
having a stepped bore 21A formed therein in which a stepped piston 22 is 
slidably fitted in a liquid tight manner. In the present embodiment, the 
control valve 8 is assembled into the stepped piston 22. 
The stepped piston 22 includes a large diameter portion 22a having an end 
face 22b, which defines a first pressure chamber 23 together with an 
opposing wall surface of the casing 21 and the wall surface of the stepped 
bore 21A which defines a large diameter bore 21a. The first pressure 
chamber 23 communites with the master cylinder 3 through an axial bore 
formed in the casing 21 and the braking liquid path 4 which is connected 
with this axial bore. In this manner, a braking liquid is introduced into 
the first pressure chamber 23, allowing the braking liquid pressure which 
is generated by the master cylinder 3 to operate on it. 
The stepped piston 22 also includes a small diameter portion 22c having an 
end face 22d, which defines a second pressure chamber 24 together with an 
opposing wall surface of the casing 21 and the wall surface of the stepped 
bore 21A which defines a small diameter bore 21b. The second pressure 
chamber 24 communicates with front wheel cylinders 5 through an axial bore 
formed in the casing 21 and its connected braking liquid path 4, whereby 
the braking liquid pressure of the front wheel cylinders 5 act upon the 
second pressure chamber 24. 
The stepped piston 22 also includes a stepped end face 22e, which defines a 
third pressure chamber 25 together with an opposing stepped wall surface 
of the stepped bore 21A and the wall surface of the large diameter bore 
21a. The third pressure chamber 25 communicates with the accumulator 12 
through a radial path 21c formed in the casing 21 and the braking liquid 
path 16 which is connected thereto, thus allowing the liquid pressure from 
the accumulator 12 to be introduced into the third pressure chamber 25. 
A return spring 26 is disposed between the stepped wall surface of the 
stepped bore 21A and the stepped end face 22e of the stepped piston 22 to 
position the stepped piston 22 at its inoperative position, shown, where 
its end face 22b abuts against the opposing wall surface of the casing 21. 
A communication path 22f axially extends through the stepped piston 22, and 
provides a communication between the first pressure chamber 23 and the 
second pressure chamber 24. 
A stepped bore 22g is formed in the axial portion of the stepped piston 22, 
and a valve member 27 including a large diameter portion 27a at its 
axially median position thereon is slidably fitted in the stepped bore 
22g. One end 27b of the valve member 27 faces the second pressure chamber 
24 while the other end 27c of the valve member 27 is located within the 
communication path 22f. A ball is embedded in the end face of the other 
end 27c of the valve member 27 in a region which is located within the 
communication path 22f, the ball defining a seat 27d. A valve seat 28 is 
formed in opposing relationship with the seat 27d, and the seat 27d is 
capable of moving toward or away from the valve seat 28. The combination 
of the seat 27d and the valve seat 28 defines an open/close valve 29 which 
opens or closes the communication path 22f. It is to be noted that the 
opposite ends 27b, 27c of the valve member 27 have an equal 
cross-sectional area d1, d2, while the seat 27d which is adapted to be 
seated upon the valve seat 28 has a cross-sectional area which is less 
than the cross-sectional area d1 of the end 27c. 
The interior of the stepped bore 22g is defined as a spring chamber 31 in a 
region which is displaced toward the second pressure chamber 24 than the 
large diameter portion 27a of the valve member 27. A spring 32 which is 
set to a given load is disposed within the spring chamber 31, thus urging 
the valve member 27 upward to close the open/close valve 29. The spring 
chamber 31 communicates with the atmosphere through a path 22h formed to 
extend through the stepped piston 22 and through a path 21d formed in the 
casing 21. Alternatively, the spring chamber 31 may communicate with the 
reservoir 14 rather than communicating to the atmosphere. 
A fourth pressure chamber 33 is defined within the interior defined by the 
stepped bore 22g in a region displaced toward the first pressure chamber 
23 than the large diameter portion 27a, and communicates with the third 
pressure chamber 25 through a path 22i formed to extend through the 
stepped piston 22. Accordingly, the pressure from the accumulator 12 is 
normally maintained within the fourth pressure chamber 33, thus urging the 
valve member 27 downward against the resilience of the spring 32 and 
maintaining the open/close valve 29 in its open condition. 
In the present embodiment, the communication path 22f, valve member 27, 
open/close valve 29, spring 32 and fourth pressure chamber 33 formed or 
disposed in the stepped piston 22 constitute together the control valve 8. 
In operation, when the liquid pressure booster 2 as well as the accumulator 
12 and the pump 13, which represent a source of liquid pressure associated 
with the booster, are normally operating, the liquid pressure from the 
accumulator 12 acts upon the fourth pressure chamber 33, whereby the 
spring 32 is compressed to maintain the valve 29 open. Under this 
condition, the valve member 27 is urged downward as governed only by the 
relationship between the pressure from the accumulator 12 which acts upon 
the fourth pressure chamber 33 to urge the valve member downward and the 
resilience of the spring 32, since the opposite ends 27b, 27c of the valve 
member 27 have equal cross-sectional areas d1, d2. Accordingly, the valve 
29 is open, whereby the first pressure chamber 23 and the second pressure 
chamber 24 communicate with each other through the communication path 22f. 
Also, a communication is established between the master cylinder 3 and the 
front wheel cylinders 5. The stepped piston 22 is located at its 
inoperative position, shown, by the return spring 26. 
When the brake pedal 1 is depressed under this condition, the booster 2 
boosts the force with which the Brake pedal 1 is depressed at a given 
ratio before it is transmitted to the master cylinder 3. This causes a 
braking liquid pressure to be generated in the master cylinder 3. This 
pressure is transmitted to the front wheel cylinders 5 through the braking 
liquid path 4 located upstream of the intensifier mechanism 7, the first 
pressure chamber 23, the communication path 22f and the second pressure 
chamber 24 of the intensifier mechanism 7, and through the braking liquid 
path 4 located downstream of the intensifier mechanism 7, thus actuating 
the brake in a normal manner. 
It will be seen that the valve 29 remains open if the brake pedal 1 
continues to be depressed after the booster ratio of the booster 2 has 
reached a full load point of 1. Accordingly, the master cylinder 3 
maintains its communication with the front wheel cylinders 5 through the 
braking liquid path 4 and the intensifier mechanism 7. Accordingly, if the 
stepped piston 22 is displaced downward from its inoperative position, 
shown, by compressing the return spring 26, the intensifier mechanism 7 
cannot be actuated. 
By contrast, in the event the accumulator 12 and the pump 13, acting as a 
source of liquid pressure, fail and the booster function by the booster 2 
is no longer available, the liquid pressure from the accumulator 12 which 
acts upon the third pressure chamber 25 and the fourth pressure chamber. 
33 will be reduced to zero or so. 
As a consequence, the resilience of the, spring 32 which urges the valve 
member 27 upward overcomes the liquid pressure from the accumulator 12, 
which has been acting upon the fourth pressure chamber 33 to urge the 
valve member 27 downward, and accordingly, the valve member 27 will be 
driven upward relative to the stepped piston 22, causing the seat 27b to 
be seated upon the valve seat 28 to close the valve 29. In this manner, a 
communication between the master cylinder 3 and the front wheel cylinders 
5 is interrupted. 
When the brake pedal 1 is depressed under this condition, it will be noted 
that a booster function by the booster 2 is no longer available, and hence 
the force with which the brake pedal 1 is depressed is directly 
transmitted to the master cylinder 3. This causes the braking liquid 
pressure to be generated in a booster 2 which acts upon the first pressure 
chamber 23 of the intensifier mechanism 7. In response thereto, the 
stepped piston 22 is forced down against the resilience of the return 
spring 26, and this brings forth the effect that the braking liquid 
pressure in the second pressure chamber 4 is intensified by an amount 
corresponding to the ratio of the cross-sectional area of the small 
diameter portion 22c to the cross-sectional area of the large diameter 
portion 22a of the stepped piston. 22, the intensified braking liquid 
pressure being transmitted to the front wheel cylinders. 
As discussed, in the present embodiment, in the event the source of liquid 
pressure associated booster 2 fails and the intensifier function by the 
booster 2 is no longer available, the liquid pressure from the accumulator 
12 which acts upon the fourth pressure chamber 33 is reduced. This allows 
the open/close valve 29 to be closed under the resilience of the spring 
32. Subsequently, when the brake pedal 1 is depressed, the intensifier 
mechanism 7 is actuated immediately. In this manner, an operational lag of 
the intensifier mechanism 7 upon failure or sinking of the booster 2 can 
be prevented. 
If the booster 2 reached its full load point during its normal operation, 
the open/close valve 29 remains open, preventing the intensifier mechanism 
7 from affording an intensifier action. Accordingly, in the 
present-embodiment, there is no need to provide a limiter in the braking 
liquid path 16 which is effective to prevent an intensifying action of the 
intensifier mechanism 7 after the booster 2 has reached its full load 
point. 
SECOND EMBODIMENT 
FIG. 3 shows a second embodiment of the invention. In contrast to the first 
embodiment in which a spring chamber 31 is open to the atmosphere or the 
reservoir 14, in the second embodiment, a spring chamber 131 is caused to 
communicate with a second pressure chamber 124 through a second open/close 
valve 140. 
Specifically, in the second embodiment, a stepped piston 122 includes a 
small diameter portion 122c, in the axial portion of which is threadably 
engaged with a plug 141 having a stepped, axial through-bore formed 
therein. The entire assembly including the plug 141 is referred to as the 
stepped piston 122. The through-bore in the plug 141 includes a large 
diameter bore 141a in which a large diameter portion 127a of a valve 
member 127 is fitted. Bore 141a defines a fourth pressure chamber 133 on 
the side of the large diameter portion 127a which is located toward a 
first pressure chamber 123, generally in the similar manner as described 
above in connection with the first embodiment. At a location below the 
large diameter portion 127a, the valve member 127 includes a stepped end 
face 127e, which defines a spring chamber 131 together with an opposing 
stepped end face 141b of the plug 141 and the wall surface of the large 
diameter bore 141a. A spring 132 tensioned to a preset load is disposed 
between the both stepped end faces 127e and 141b to urge the valve member 
127 upward. 
The valve member 127 includes an end 127b located within the spring chamber 
131 and which has an end face, in which a ball is embedded to form a seat 
127f. This ball is of the same diameter-as the diameter of the other ball 
which is used to form an other open/close valve 129. The plug 141 includes 
a small diameter bore 141c which is contiguous to the large diameter bore 
141a. At a boundary between the both bores 141a and 141c, a valve seat 142 
is formed thereon so as to be located opposite to the seat 127f to permit 
the latter to move toward or away therefrom. The combination of the seat 
127f and the valve seat 142 defines a second open/close valve 140. 
The opposite ends 127b, 127c of the valve member 127 have an equal 
cross-sectional area, and the both seats 127b, 127f also have an equal 
cross-sectional area. 
In this embodiment, a return spring 126 is disposed within a second 
pressure chamber 124. 
In other respects, the arrangement is similar to that of the first 
embodiment, and accordingly, corresponding parts are designated by like 
numerals as used before in the description of the first embodiment, to 
which 100 is added. 
In operation, when a liquid pressure booster and an associated source of 
liquid pressure such as accumulator 112 are operating in a normal manner, 
a liquid pressure from an accumulator 112 acts on the fourth pressure 
chamber 133, whereby the spring 132 is compressed to close the valve 140 
while the other valve 129 is open. The stepped piston 122 is positioned at 
its inoperative position, shown, by the return spring 126. 
When the brake pedal is depressed under this condition, the liquid pressure 
booster boosts a force with which the brake pedal is depressed at a given 
ratio before it is transmitted to a master cylinder 103, and the braking 
liquid pressure generated therein is transmitted to front wheel cylinders 
without being intensified by an intensifier mechanism 107. Thus, a normal 
braking operation takes place. 
If the brake pedal continues to be depressed after the booster ratio of the 
liquid pressure booster has reached its full load point of 1, the valve 
129 remains open, and hence, if the stepped piston 122 compresses the 
return spring 126 to slide downward within the stepped bore 121A, there 
cannot occur an actuation of the booster mechanism 107. 
However, when the accumulator 112 and an associated pump, acting as a 
source of liquid pressure, fail and a booster function by the liquid 
pressure booster is no longer available, the liquid pressure from the 
accumulator 112 which acts on the third pressure chamber 125 and the 
fourth pressure chamber 133 will be reduced to zero or so. 
Accordingly, the resilience of the spring 132 which urges the valve member 
127 upward exceeds the effect of the liquid pressure from the accumulator 
112 which has been depressing the valve member 127, and accordingly, the 
valve member 127 is driven upward against the stepped piston 122, allowing 
the seat 127d to be seated on the valve seat 128 to close the valve 129, 
while opening the second open/close valve 140. 
As a consequence, a communication between the first pressure chamber 123 
and a second pressure chamber 124 is interrupted, as is a communication 
between the master cylinder 103 and front wheel cylinders 105. The opening 
of the second valve 140 allows a communication between the spring chamber 
131 and the second pressure chamber 124, whereby the liquid pressure in 
the second pressure chamber 124 (or the liquid pressure in the front wheel 
cylinders 105) acts on the valve member 127. Consequently, the valve 
member 127 is subjected not only to the resilience of the spring 132, but 
also to the liquid pressure from the third pressure chamber 125, whereby 
the open/close valve 129 is closed in a reliable manner. 
When the brake pedal is depressed under this condition, the braking liquid 
pressure from the master cylinder 103 is intensified by the intensifier 
mechanism 107 for transmission to the front wheel cylinders 105, in the 
similar manner as described above in connection with the first embodiment. 
Accordingly, a similar functioning and effects are achieved as in the 
previous embodiment. 
THIRD EMBODIMENT 
FIG. 4 shows a third embodiment of the invention. Briefly, in the third 
embodiment, the communication path 22f shown in the first embodiment 
indicated in FIG. 2 is formed in the axial portion of the valve member 27, 
together with concomittant improvements. 
Specifically, a sliding member 227, which corresponds to valve member 27 in 
the first embodiment, includes a large diameter portion 227a toward its 
center, as viewed in the axial direction thereof, and also includes a 
medium diameter portion 227b immediately above the large diameter portion 
227a. Opposite ends of the sliding member 227 have an equal diameter which 
is less than that of the medium diameter portion 227b. The lower portion 
of the sliding member 227 is disposed in a communication channel 222f 
formed in the axial portion of a stepped piston 222, and the bottom end 
227c of the sliding member 227 faces a second pressure chamber 224. The 
top end 227d of the sliding member 227 extends through the communication 
path 222f to face a first pressure chamber 223. 
A communication path 227e axially extends through the sliding member 227 in 
alignment with the axis thereof, and has its diameter increased at its top 
end which faces the first pressure chamber 223. A spherical valve element 
228 is received in the top portion of the path 227e having an increased 
diameter so that the valve element 228 is vertically movable. A first 
spring 231 is disposed in this top end to urge the valve element 228 in a 
direction away from the communication path 222f (or toward the first 
pressure chamber 223). 
An arcuate stop member 232 is fitted around the inner periphery of the top 
end of the communication path 227e having an increased diameter. The inner 
periphery of the stop member 232 is tapered so that its diameter decreases 
toward the top end. Accordingly, the valve element 228 which is urged 
upward by the first spring 231 is urged into abutment against the inner 
periphery of the stop member 232, thus preventing its disengagement and 
restricting a seat portion thereof which is located above the stop member 
to a given elevation. If the valve element 228 abuts against the inner 
periphery of the stop member 232, an axial clearance formed in the stop 
member 232 allows a communication between the communication path 222f, at 
a location immediately adjacent to the valve element 228, and the second 
pressure chamber 224 through the communication path 227e. In this manner, 
the stop member 232 functions to prevent the projection of the valve 
element 228 from the communication path 227e beyond a given amount and to 
maintain a communication across the stop member 232. 
At a location opposite to the valve element 228, the communication path 
222f is formed with a step, the inner peripheral surface of which defines 
an annular valve seat 233 which may be engaged by the valve element 228. 
In this manner, an open/close valve 234 is defined by the valve element 
228 and the valve seat 233 to open or close the communication path 222f. 
A spring chamber 235 is defined within the communication path 222f at a 
location displaced from the large diameter portion 227a of the sliding 
member 227 toward the second pressure chamber 224, and a second spring 236 
which is tensioned to a preset load is disposed within the spring chamber 
235. The resilience of the second spring 236 normally urges the sliding 
member 227 upward within the communication path 222f. 
The spring chamber 235 communicates with a reservoir 214 through a path 
222g formed in the stepped piston 222, a path 221f formed in a casing 221, 
and through a braking liquid path, not shown, which is connected to the 
path 221f. Alterantively, the spring chamber 235 may be open to the 
atmosphere rather than making it to communicate with the reservoir 214. 
A fourth pressure chamber 237 is defined above the large diameter portion 
227a of the sliding member 227, and communicates with a third pressure 
chamber 225 through a path 222h formed in the stepped piston 222. 
Consequently, a liquid pressure from an accumulator 212 acts on the fourth 
pressure chamber 237. This causes the sliding member 227 to be urged 
downward within the communication path 222f against the resilience of the 
second spring 236, and comes to a stop at its inoperative position, shown, 
where the lower stepped end face of the large diameter portion 227a abuts 
against an opposing stepped end face 222f of the communication path 222f. 
When the sliding member 227 remains stationary at its inoperative position 
under the action of the liquid pressure in the fourth pressure chamber 
237, the seat of the valve element 228 urged by the first spring 231 which 
projects above the stop member 232 tends to be seated upon the valve seat 
233. Accordingly, in the present embodiment, one end of a pin-shaped 
engaging member 238 is fitted in a path 221b formed in the casing 221 in a 
region of the first pressure chamber 223 so that the engaging member 238 
lies in the first pressure chamber 223 with its distal end located 
opposite to the valve element 228. When the stepped piston 222 assumes its 
inoperative position, shown, the distal end of the engaging member 238 
extends into the communication path 222f from above to abut against the 
valve element 228. This action forces forcing the valve element 228 down 
against the resilience of the first spring 231 to open the valve 234. It 
is to be noted that said one end of the engaging member 238 which is 
fitted in the path 221b is formed with a plurality of axially extending 
bores to permit a flow of the braking liquid from the master cylinder 203. 
In this manner, in the operative position shown, the first pressure 
chamber 223 and the second pressure chamber 224 communicate to each other, 
and a communication is established between the master cylinder 203 and 
front wheel cylinders 205. 
In addition, in the third embodiment, the large diameter portion 222a of 
the stepped piston 222 is formed with an annular groove 222i, which 
defines a reservoir 239 for braking liquid together with the inner wall 
surface of the large diameter bore 221a. Another reservoir 239' is formed 
as an annular space immediately above the medium diameter portion 227b of 
the sliding member 227. Reservoirs 239, 239' communicate with each other 
through a recovery path 222j formed in the stepped piston 222, and also 
communicate with the path 222g which in turn communicates with the 
reservoir 214. In this manner, any residue of braking liquid which remains 
in the sliding portion of the large diameter portion 222a of the stepped 
piston 222 is temporarily stored in the reservoir 239', and subsequently 
the braking liquid in these reservoirs 239 , 239' are recovered to the 
reservoir 214 through the recovery passage 222j, the path 222g and the 
like. In this manner, a loss of braking liquid can be prevented. 
A communication between the both reservoirs 239, 239' and the reservoir 214 
is established by the recovery passage 222j which extends both vertically 
and substantially radially within the stepped piston 222. However, 
alternatively, a path which allows the communication between reservoirs 
239, 239' may be formed within the stepped piston 222, and another path 
which allows a communication between the reservoir 239 and the path 221f 
formed in the casing 221 may be formed within the casing 221. This 
dispenses with the formation of a vertical path within the stepped piston 
222, allowing a reduction in the size of the stepped piston 222, and hence 
of a booster mechanism 207. 
Finally, in the third embodiment, seal means 251 and 252 maintans a liquid 
tightness between the stepped piston 222 and a stepped bore 221A formed in 
the casing 221 is constructed as follows: seal means 251 provided on the 
large diameter portion 222a of the stepped piston 222 for maintaining a 
liquid tightness between the large diameter portion 222a and the large 
diameter bore 221a comprises an annular, first seal member 253 mounted 
around the outer peripheral surface of the large diameter portion 222a at 
a location displaced from the annular groove 222i toward the first 
pressure chamber 223, and an annular. A second seal member 254 is mounted 
around the outer peripheral surface of the large diameter portion at a 
location displaced from the annular groove 222i toward the third pressure 
chamber 225. The first seal member 253 is formed of rubber, and is 
cup-shaped in section. On the other hand, the second seal member 254 
contains an O-ring on the inside, the outside of which is covered by a 
Teflon ring. 
Seal means 252 which is provided on the small diameter portion 222c of the 
stepped piston 222 for maintaining a liquid tightness between the small 
diameter portion 222c and the small diameter bore 221c comprises, an 
annular, first seal member 253' mounted around the outer peripheral 
surface of the small diameter portion 222c at a location displaced from 
the path 222g toward the second pressure chamber 224. Seal mean 252 also 
includes an annular, second seal member 254' mounted around the outer 
peripheral surface of the small diameter portion 222c at a location 
displaced from the path 222g toward the third pressure chamber 225. The 
first seal member 253' is made of rubber and is cup-shaped in section, in 
the similar manner as the first seal member 253, and the second seal 
member 254 contains an O-ring on the inside, the outside of which is 
covered by a Teflon ring, in the similar manner as the second seal member 
254. 
The operation of the third embodiment mentioned above will now be 
described. When the liquid pressure booster, the accumulator 212 and the 
pump are all operating in a normal manner, the liquid pressure from the 
accumulator 212 operates on the third pressure chamber 225 and the fourth 
pressure chamber 237 to compress the second spring 236. Accordingly, the 
sliding member 227 assumes its inoperative position, shown, where the 
stepped end face of the large diameter portion 227a abuts against the 
stepped end face 222f' of the communication path 222f. The stepped piston 
222 is located at its-inoperative position, shown, by the resilience of 
the return spring 226. Accordingly, the valve element 228 which is located 
at the top end of the sliding member 227 abuts against the distal end of 
the engaging member 238 and hence is depressed, thus making the valve 234 
open. 
When the brake pedal is depressed under this inoperative condition, the 
liquid pressure booster boosts a force with which the brake pedal is 
depressed at a given ratio before it is transmitted to the master cylinder 
203. A braking liquid pressure is generated within the master cylinder 203 
and acts on the first pressure chamber 223 of the intensifier mechanism 
207. Since the valve 234 is open at this time, the intensifier mechanism 
207 is not actuated, and the liquid pressure acting in the first pressure 
chamber 223 is conveyed to the second pressure chamber 224 through the 
communication paths 222f, 227e, and thence fed to the front wheel 
cylinders 205. In this manner, a normal braking operation takes place. 
When the brake pedal continues to be depressed after the booster ratio of 
the liquid pressure booster has reached its full load point of 1 under the 
normal condition of the booster and its associated source of liquid 
pressure, the liquid pressure in the accumulator 212 which acts upon the 
third pressure chamber 225 remains constant while the liquid pressure in 
the master cylinder 203 which acts upon the first pressure chamber 223 
rises and exceeds the level of the liquid pressure within the third 
pressure chamber 225. Accordingly, the stepped piston 222 will be driven 
downward from its inoperative position, shown, against the resilience of 
the return spring 226. 
Concomittantly, the valve element 228 which then moves downward, moves away 
from the distal end of the engaging member 238. However, because the 
fourth pressure chamber 237 is subject to the action of the liquid 
pressure from the accumulator 212, the sliding member 227 remains at rest 
within the communication path 222f while maintaining its inoperative 
position shown. On the other hand, the valve element 228 which is urged by 
the first spring 231, the amount of projection of which in the outward 
direction is constrained by abutment against the stop member 232, will be 
removed from the valve seat 233 to open the valve 234. Accordingly, 
subsequent to the full load point of the liquid pressure booster, the 
intensifier mechanism 207 cannot be actuated, and hence an intensifying 
action thereof is unavailable despite the stepped piston 222 moves 
downward from its inoperative position shown. 
In this-manner, it will be seen that in the third embodiment, the 
intensifier mechanism 207 is not actuated when the liquid pressure in the 
master cylinder 203 rises above the liquid pressures of the third pressure 
chamber 225 and the fourth pressure chamber 237 subsequent to the full 
load point. Accordingly, there is no need to provide a limiter in the 
braking liquid path 216 in order to prevent the intensifier mechanism 207 
from being actuated subsequent to the full load point, thus simplifying 
the arrangement. 
In contrast to the normal operation mentioned above, when the accumulator 
212 and the pump, which represent the source of liquid pressure associated 
with the booster, fail and a booster action by the liquid pressure booster 
is no longer available, the liquid pressure in the accumulator 212 which 
acts upon the third pressure chamber 225 and the fourth pressure chamber 
237 will be reduced to zero or so. Accordingly, the resilience of the 
second spring 236 which urges the sliding member 227 upward overcomes the 
liquid pressure from the accumulator 212 which has been acting upon the 
fourth pressure chamber 237 to depress the sliding member 227. Hence, the 
sliding member 227 will rise upward from its inoperative position shown, 
but since the stepped piston 222 is in its inoperative position shown at 
this time, the valve element 228 stays in abutment against the distal end 
of the engaging member 238, thus opening the valve 234. 
When the brake pedal is now depressed, the absence of the booster action 
from the liquid pressure booster causes the force with which the brake 
pedal has been depressed to be directly transmitted to the master cylinder 
203 without experiencing a booster action by the booster. The liquid 
pressure in the master cylinder 203 then rises, and acts upon the first 
pressure chamber 223 in the intensifier mechanism 207. 
When the liquid pressure from the master cylinder 203 acts upon the first 
pressure chamber 223, the stepped piston 222 is driven downward from its 
inoperative position, shown, against the resilience of the return spring 
226 since the liquid pressure in the third pressure chamber 225 is equal 
to zero. Thus the valve element 228 moves away from the distal end of the 
engaging member 238. Subsequently, since the sliding member 227 is raised 
by the second spring 236 and the valve element 228 is urged upward by the 
first spring 231, the seat of the valve 228 which projects above the stop 
member 237 becomes seated upon the valve seat 233 to close the valve 234. 
This interrupts a communication between the master cylinder 203 and the 
front wheel cylinders 205. In the present embodiment, if a failure of the 
accumulator 212 has already occurred at the time the brake pedal is 
depressed, there is a slight time lag from the commencement of depression 
of the brake pedal to the closure of the valve 234. During this time 
interval, an amount of braking liquid which corresponds to a lost motion 
of the front wheel cylinders 205 can be supplied to these cylinders 205. 
In the event a failure of the accumulator 212 occurs after the brake pedal 
1 is depressed under a normal condition of the accumulator 212 to provide 
a normal braking operation, the valve 234 is closed in the manner 
mentioned above. However, during a short time lag which occurs from the 
time when the third pressure chamber 225 falls to a liquid pressure of 
zero to the closure of the valve 234, the braking liquid in the second 
pressure chamber 224 flows into the first pressure chamber 223 through the 
communication paths 222f, 227e. In this manner, the braking liquid of a 
high liquid pressure is prevented from remaining within the braking liquid 
path 204 leading to the front wheel cylinders 205, thus preventing a 
dragging of the brake. 
Subsequent to the closure of the valve 234, as the brake pedal continues to 
be depressed, the stepped piston 222 will be driven downward against the 
resilience of the return spring 226, and the braking liquid pressure in 
the second pressure chamber 224 will be intensified by an amount 
corresponding to the ratio of the cross-sectional area of the small 
diameter portion 222c to the cross-sectional area of the large diameter 
portion 222a of the stepped piston 222 to be transmitted to the front 
wheel cylinders 205. 
When the brake pedal is released, the liquid pressure in the master 
cylinder 203 reduces, and accordingly, the stepped piston 222 is returned 
to its inoperative position shown under the resilience of the return 
spring 226. The valve element 228 will then abuts against the engaging 
member 238 to open the valve 234. 
According to the third embodiment described above, if the liquid pressure 
booster has reached its full load point under the normal condition, the 
intensifying action by the intensifier mechanism 207 is not available 
since the valve 234 thereof is opened. This avoids a need for the 
provision of a limiter in the braking liquid path 216 for preventing a 
intensifying action by the intensifier mechanism 207 after the liquid 
pressure booster has reached its full load point. In addition, the braking 
liquid path 204 extending between the second pressure chamber 224 and the 
front wheel cylinders 205 may be designed to present a low pressure 
withstanding capability. 
In addition, in this embodiment, the seal means 251, 252 mounted on the 
stepped piston 222 each comprises pairs of seal members 253, 254, and 
253', 254', which provides an additional advantage. Specifically, if the 
first seal member 253 of the seal means 251 fails to maintain a liquid 
tightness, the braking liquid from the first pressure chamber 223 will 
leak to the reservoir 239 through the failing portion of the first seal 
member 253, and thence to the reservoir 204 through the recovery path 222j 
or the like. However, the liquid pressure from the accumulator 212 which 
is acting upon the third pressure chamber 225 can be maintained at its 
normal value since the liquid tightness is maintained by the second seal 
member 254, thus securing a normal booster action of the liquid pressure 
booster. Accordingly, in the event the first seal member 253 for the 
intensifier mechanism 207 associated with the front wheels fails, the 
liquid pressure booster still operates in a normal manner when the brake 
is operated, assuring a normal operation of the brake associated with the 
rear wheels. The same applies when the first seal member 253' of the seal 
means 252 mounted around the small diameter portion 222c should fail. 
Specifically, when the first seal member 253' fails to maintain the liquid 
tightness, the braking liquid within the second pressure chamber 224 will 
leak to the recovery path 222g through the failing portion of the first 
seal member 253', and thence to the reservoir 214 through the path 221f or 
the like. However, the other or second seal member 254' maintains the 
liquid tightness, whereby the liquid pressure from the accumulator 212 
which is acting upon the third pressure chamber 225 is maintained at its 
normal value, assuring a normal booster action of the liquid pressure 
booster 2. 
On the other hand, if the second seal member 254 of the seal means 251 
fails to maintain the liquid tightness, the braking liquid which is 
introduced into the third pressure chamber 225 will leak to the reservoir 
239 through the failing portion of the second seal member 254, and thence 
to the reservoir 214 through the recovery path 222j or the like. 
Accordingly, the liquid pressure in the accumulator 212 will be reduced to 
degrade the booster function of the liquid pressure booster, but the other 
or first seal member 253 maintains the liquid tightness, thus assuring a 
normal intensifying function of the intensifier mechanism 207. 
Accordingly, if the second seal member 254 of the intensifier mechanism 
207 associated with the front wheels, for example, fail, the booster 
function of the liquid pressure booster will be reduced upon operation of 
the brake, but the intensifier mechanism 207 associated with the front and 
the rear wheels operate in a normal manner, and the braking liquid 
pressure produced by the master cylinder 203 is intensified by the 
intensifier mechanism 207 to be transmitted to the front and the rear 
wheel cylinders. The same applies when the second seal member 254' of the 
seal means 252 mounted around the small diameter portion 222c should fail. 
Thus, if the second seal member 254' fails to maintain the liquid 
tightness, the braking liquid within the third pressure chamber 225 will 
leak to the recovery path 222g through the failing portion of the second 
seal member 254, and thence to the reservoir 214 through the path 221f or 
the like, thus degrading the booster function of the liquid pressure 
booster. However, since the other or first seal member 253 maintains the 
liquid tightness, the intensifier mechanism 207 will operate in a normal 
manner. 
The effect of providing seal means 251, 252 has been described above in 
connection with an embodiment in which the liquid pressure from the 
accumulator 212 is acting upon the third pressure chamber 225, but a 
similar effect can be obtained when the liquid pressure from the power 
chamber of the liquid pressure booster is acting upon the third pressure 
chamber 225. 
FOURTH EMBODIMENT 
FIG. 5 shows a fourth embodiment of the invention. In each embodiment 
described above, the control valve 8 (108, 208) is assembled into the 
stepped piston 22 (122, 222). However, in the fourth embodiment, a 
switching valve 350, which forms part of a control valve 308, is disposed 
outside a casing 321. 
Specifically, in the fourth embodiment, a single stepped through-bore 322f 
is formed in the axial portion of a stepped piston 322 and serves as a 
communication path between a first pressure chamber 323 and a second 
pressure chamber 324. The bore 322f has a portion of an increased diameter 
which contains a spherical valve element 327, and a valve sleeve 328 is 
disposed in opposing relationship therewith, the valve element 327 moving 
toward and away from the valve sleeve to define an open/close valve 329. A 
projection 321e is formed on the wall surface of the casing 321 which 
faces the first pressure chamber 323, with the distal end of the 
projection 321e being disposed opposite to the valve element 327. 
When an intensifier mechanism 307 is inoperative, an end face 322b of a 
large diameter portion 322a of the stepped piston. 322 abuts against the 
wall surface of the casing 321 under the resilience of a return spring 
326, whereby the distal end of the projection 321e extends through the 
valve seat 328 to keep the valve element 327 removed from the valve seat 
328. Thus, the valve 329 is opened providing a communication between a 
master cylinder 303 and front wheel cylinders 305. 
In the fourth embodiment, a liquid pressure from the master cylinder 303 is 
introduced into a third pressure chamber 325 through a braking liquid path 
351, which is in turn connected to an end of a braking liquid path 352 
communicating with a reservoir 314 through the switching valve 350. 
Alternatively, an accumulator 312 may be connected to the braking liquid 
path 352, instead of the reservoir 314, or a liquid pressure booster may 
be connected in turn. Rather than introducing the liquid pressure from the 
master cylinder 303 into the third pressure chamber 325, the liquid 
pressure from the front wheel cylinders 305 may be introduced into the 
third pressure chamber 325. 
The switching valve 350 is slidably disposed in a housing, not shown, and 
is urged upward by a spring 353 which is tensioned to a preset load while 
the liquid pressure from the accumulator 312 which is introduced into the 
housing urges the valve in the opposite direction from the spring 353. 
In this embodiment, a combination of the through-bore 322f in the stepped 
piston 322, the valve 329 and the switching valve 350 forms a control 
valve 308. 
In other respects, the arrangement is similar to that of the first 
mentioned embodiment, and accordingly corresponding parts are designated 
by like numerals as used before, to which 300 is added, while omitting 
their description. 
Describing the operation of the fourth embodiment, when the liquid pressure 
booster and its associated source of liquid pressure, namely, the 
accumulator 312 or the like, are operating in a normal manner, the liquid 
pressure from the accumulator 312 acts on the switching valve 350 to 
compress the spring 353 as shown in FIG. 5. Accordingly, the liquid 
pressure from the master cylinder 303 is introduced into the third 
pressure chamber 325 through the braking liquid path 351 and the switching 
valve 350, in addition to the first pressure chamber 323 and the second 
pressure chamber 324. 
Under this condition, the upside and the downside of the stepped piston 322 
is subject to the same pressure, namely, the liquid pressure from the 
master cylinder 303, and accordingly, the stepped piston 322 is in its 
inoperative position shown, and the valve 329 is open. Hence, a 
communication is established between the master cylinder 303 and the front 
wheel cylinders 305, and when a brake pedal is depressed under this 
condition, a normal brake operation takes place. 
In contrast to the normal condition mentioned above, when the accumulator 
312 and the associated pump, which represents a source of liquid pressure, 
fail and a booster action by the liquid pressure booster is no longer 
available, the liquid pressure from the accumulator 312 which acts upon 
the switching valve 350 will be reduced to zero or so, whereby the 
resilience of the spring 353 exceeds the depressing action of the liquid 
pressure, thus switching a flow path in the switching valve 350. As a 
consequence, a communication between the third pressure chamber 325 and 
the master cylinder 303 is interrupted, while a communication is 
established between the third pressure chamber 325 and the reservoir 314, 
reducing the liquid pressure within the fourth pressure chamber 325 to 
zero. 
Accordingly, the stepped piston 322 is driven downward against the 
resilience of the return spring 326 under the influence of the liquid 
pressure from the master cylinder 303 which acts upon the first pressure 
chamber 323, and simultaneously the valve element 327 will be removed from 
the distal end of the projection 321e and will be seated upon the valve 
seat 328 to close the valve 329. This interrupts a communication between 
the first pressure chamber 323 and the second pressure chamber 324, and 
hence also a communication between the master cylinder 303 and the front 
wheel cylinders 305. Accordingly, if the brake pedal is now depressed, the 
braking liquid pressure from the master cylinder 303 will be intensified 
by the intensifier mechanism 307 to be transmitted to the front wheel 
cylinders 305 in the same manner as mentioned previously in connection 
with the first embodiment. The described arrangement of the fourth 
embodiment achieves the similar functioning and effects as achieved by the 
first and the second embodiment. 
In the first and the second embodiment, the liquid pressure from the 
accumulator 12 or 112 is introduced into the fourth pressure chamber 33 or 
133 through the third pressure chamber 25 or 125, but instead of the 
liquid pressure from the accumulator, the liquid pressure from the liquid 
pressure booster may be introduced into the such pressure chamber. Also, 
in the fourth embodiment, the liquid pressure from the accumulator 312 
which is chosen to act upon the switching valve 350 may be replaced by the 
liquid pressure from the liquid pressure booster. 
In such instance, a preset load upon the spring 32, 132 or 353 is reduced 
than the load chosen when introducing the liquid pressure from the 
accumulator so that the control valve 8, 108 or 308 is maintained in its 
inoperative position by the pressure in the power chamber which prevails 
immediately after the commencement of the booster action by the liquid 
pressure booster. Accordingly, when the liquid pressure booster is 
inoperative, the control valve 8, 108 or 103 is in its operative position, 
but during a normal operation, as soon as the liquid pressure booster 
operates, the liquid pressure in the power chamber is introduced into the 
third pressure chamber 25, 125 and the fourth pressure chamber 33, 133 in 
the first and the second embodiment. Consequently, the control valve 8 or 
108 moves to its inoperative position, closing the valve 29 or 129, which 
prevents the intensifier mechanism 7 or 107 from being actuated. In the 
fourth embodiment, the switching valve 350 is switched to introduce the 
master cylinder pressure or the wheel cylinder pressure into the third 
pressure chamber 325, again preventing an actuation of the intensifier 
mechanism 307. If the source of liquid pressure fails and the accumulator 
pressure becomes equal to zero, the liquid pressure in the power chamber 
will be zero if the liquid pressure booster operates, and since the 
control valve 8, 108 or 308 remains in its operative position, the 
intensifier mechanism is actuated in each of the first, the second and the 
fourth embodiment. 
While the invention has been shown and disclosed above in connection with 
several embodiments thereof, it should be understood that a number of 
changes, modifications and substitutions therein will readily occur to one 
skilled in the art from the above disclosure without departing from the 
spirit and scope of the invention as defined by the appended claims.