Braking force holding device for an air brake system

A braking force holding device for an air brake system, maintains a braking force when a vehicle is stopped facing upwardly on a hill to facilitate the start of the vehicle on the hill. The braking force holding device includes a main body having an inlet port connected to a brake valve, an outlet port connected to a pneumatic relay valve, a control port connected to the pneumatic actuator of a clutch operating device, and a passage interconnecting the inlet port and the outlet port; a valve device capable of intercepting the flow of air from the outlet port toward the inlet port; and a control unit for controlling the valve device. The control unit has a movable member capable of being shifted to a first position to open the valve device or to a second position to close the valve device depending on the difference between a force corresponding to the pressure of compressed air applied thereto through the control port and a force corresponding to the pressure of compressed air applied thereto through the outlet port. The timing of relieving the braking pressure maintained is varied according to the level of the braking pressure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
The present invention relates to a braking force holding device for a 
vehicle equipped with an air brake system, intended for facilitating the 
start of the vehicle on an upward hill. 
2. Description of the Prior Art: 
Japanese Laid-Open Patent Publication No. 58-105867, for instance, 
discloses a braking force holding device comprising a main body having an 
inlet port connected to a brake valve which discharges compressed air when 
operated by the driver, an outlet port connected to a brake actuator which 
is actuated by compressed air, and a passage interconnecting the inlet 
port and the outlet port, a first valve provided in the passage, capable 
of detecting the hill-climbing condition of the vehicle, and capable of 
closing upon the detection of the hill-climbing condition to intercept the 
flow of air from the outlet port to the inlet port, and a second valve 
provided in the passage in parallel to the first valve, having a valve 
element capable of being moved in the opening direction by a pressure 
applied thereto on the side of the inlet port and the resilience of a 
valve spring, and capable of being moved in the closing direction by a 
liquid pressure applied thereto by a clutch actuator to intercept the flow 
of air from the outlet port to the inlet port. 
In operation, the first valve closes when the vehicle comes to a hill. 
Then, when the clutch actuator is operated to disengage the clutch after 
supplying compressed air to the brake actuator by operating the brake 
valve to stop the vehicle, liquid pressure produced by the clutch actuator 
shifts the valve element of the second valve against the pressure applied 
to the valve element on the side of the inlet port and the resilience of 
the valve spring to close the second valve. Thus, both the first and 
second valves are closed to disconnect the inlet port and the outlet port. 
Consequently, the pressure applied to the brake actuator is maintained 
even if the operation of the brake valve is cancelled after the vehicle 
has been braked to a stop as long as the clutch is held disengaged, so 
that the braking force is maintained to hold the vehicle stopped. During 
the starting of the vehicle, when the clutch engaging operation is 
performed in combination with the operation of the accelerator, the effect 
of the liquid pressure applied to the valve element by the clutch actuator 
decreases below the resilience of the valve spring to allow the valve 
element to be shifted in the opening direction and hence, the second valve 
is opened. Thus, the braking pressure maintained in the brake actuator is 
relieved as the driving force of the engine is transmitted through the 
clutch to the driving wheels. Accordingly, the vehicle can be easily 
started on a hill without requiring simultaneous dexterous manipulation of 
the hand brake, the accelelator and the clutch. 
In this known braking force holding device, the second valve is opened to 
start relieving the brake holding pressure after the liquid pressure 
applied to the second valve by the clutch actuator has dropped to a fixed 
level below the resilience of the valve spring and the frictional 
engagement of the clutch has been enhanced to a fixed degree. Thus, the 
opening timing of the second valve is dependent on the resilience of the 
valve spring regardless of the level of the brake holding pressure. On the 
other hand, in an air brake system employing compressed air as a working 
fluid, the pressure reduction rate, namely, pressure reduction per unit 
time, is comparatively high at the moment of actuation of the brake 
pressure relief valve. However, the pressure reduction rate decreases with 
time. Therefore, the air brake system requires a comparatively long time, 
as compared with a hydraulic brake system, for sufficiently relieving the 
brake pressure. Such a characteristic of the air brake system is enhanced 
when the brake pressure is high. 
Accordingly, when the grade of the hill or the weight of the vehicle is 
large and the brake pressure is maintained at a high level while the 
vehicle is stopped, the relief of the brake pressure is delayed relative 
to the degree of frictional engagement of the clutch according to the 
progress of clutch engaging operation. Therefore, the brake pressure is 
not reduced according to the increase of the driving force transmitted to 
the driving wheels through the clutch, and hence, an excessive braking 
force works on the vehicle against the driving force. Consequently, the 
drag of the brakes, the excessive slip of the clutch and engine stop are 
likely to occur. 
Thus, the conventional braking force holding device causes the rapid 
abrasion of the clutch facing and the brake linings or impedes the smooth 
start of the vehicle, when the braking pressure is maintained at a high 
level. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a braking 
force holding device for an air brake system, capable of varying brake 
releasing timing according to the level of the braking pressure maintained 
so that the rapid wear of the frictional members of the brakes and the 
clutch due to delay in releasing the brakes and the vehicle can be easily 
and smoothly started when on a hill and facing upwardly. 
In order to achieve the object of the invention, the present invention 
provides a braking force holding device for an air brake system, 
comprising: a main body having an inlet port connected to a main brake 
control device, such as a brake valve, which discharges compressed air 
when operated by the driver, an outlet port connected to a driven device, 
such as a brake relay valve, which is actuated by compressed air, a 
control port connected to the actuator of a clutch operating device which 
is actuated by compressed air, and a passage interconnecting the inlet 
port and the outlet port; a valve device disposed in the passage, and 
capable of intercepting the flow of air from the outlet port toward the 
inlet port; and a control unit having a movable member capable of being 
shifted toward a first position for opening the valve device by a pressure 
applied to the outlet port, and being shifted toward a second position for 
closing the valve device by a pressure applied to the control port. 
The movable member of the control unit is moved between the first position 
and the second position according to the difference between the pressure 
applied to the clutch operating device and the pressure maintained in the 
driven device, to control the valve device. During the release of the 
brakes in response to clutch engaging operation, when a force 
corresponding to the pressure produced by the clutch operating device, 
which decreases with the progressive engagement of the clutch, becomes 
smaller than a force corresponding to the braking pressure maintained in 
the driven device, the movable member is shifted from the second position 
to the first position to open the valve device. Since the higher the 
braking pressure maintained in the drive device, the lower the reduction 
rate of the pressure of the clutch operating device, the movable member 
starts moving toward the first position to open the valve device while the 
degree of frictional engagement of the clutch is small, so that braking 
pressure relieving timing is advanced. Thus, even when a high braking 
force is held, the braking pressure is reduced to a level capable of 
providing a braking force necessary for restraining the vehicle from 
backward movement, when the frictional engagement of the clutch is 
enhanced to a degree where the clutch is in slipping engagement. 
Consequently, the braking pressure is reduced at an appropriate rate 
according to the increase of the driving force transmitted through the 
clutch to the driving wheels. 
As is apparent from the foregoing description, the present invention 
prevents the rapid wear of the frictional members of the brakes and the 
clutch due to the delayed release of the brakes, and enables the vehicle 
to be started easily and smoothly on a hill when facing upward without the 
drag of the brakes and the possibility of engine stop. 
Furthermore, according to the present invention, the actuator of the clutch 
operating device is operated by compressed air and does not have any part 
operated by liquid pressure, which is higher than the pressure of the 
compressed air. Therefore, conditions of pressure tightness and strength 
of the braking force holding, device according to the present invention 
are reduced as compared with the conventional braking force holding device 
employing liquid pressure working in the clutch operating device for 
controlling the valve unit. Accordingly, the present invention improves 
the productivity of the production line for producing the braking force 
holding device, and enhances the reliability of the braking force holding 
device. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description taken 
in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, indicated generally at 1 is a braking force holding 
device, in a first embodiment, according to the present invention, and at 
2 is a main body. A large diameter bore 3 is formed horizontally in the 
lower part of the main body 2 so as to be open to the left side surface, 
as viewed in FIG. 1, of the main body 2. An inlet fitting 7 having an 
inlet port 9 and an inlet passage 10 connecting the inlet port 9 to the 
large diameter bore 3 is screwed in the large diameter bore 3. A sealing 
ring is provided between the head of the inlet fitting 7 and the main body 
3 to seal the large diameter bore 3 airtightly. A stepped vertical bore 4 
having a lower large diameter section 5 opening to the lower surface of 
the main body 2 is formed vertically, as viewed in FIG. 1, through the 
main body 2. The large diameter bore 3 communicates with the large 
diameter section 5 of the vertical bore 4 by means of a through hole 6. An 
outlet fitting 8 is screwed in the lower opening of the large diameter 
section 5, and a sealing ring is provided between the head of the outlet 
fitting 8 and the main body 2 to seal the large diameter section 5 
airtightly. The outlet fitting 8 has an outlet port 11 and an outlet 
passage 12 interconnecting the outlet port 11 and the vertical bore 4. 
A substantially cylindrical guide member 13 made of a synthetic resin and 
having a radially inwardly extending brim is fitted in the inner portion 
of the large diameter bore 3 with the brim thereof disposed on the side of 
the inlet port 9. A plurality of axial ridges 15 are formed in the inner 
circumference of the guide member 13. A ball 14 is contained within the 
guide member 13 so as to be guided by the axial ridges 15 for free 
movement within the guide member 13. A retainer 16 having a plurality of 
tongues 17 is disposed between the guide member 13 and the inner end of 
the large diameter bore 3. When the ball 14 is moved under its own weight 
rightward, as viewed in FIG. 1, the ball 14 abuts against the tongues 17. 
The outer end of the guide member 13 communicates with the through hole 6 
by means of furrows formed between the axial ridges 15 and the inside 
opening of the retainer 16. 
An annular supporting member 18 provided with a sealing ring in the outer 
circumference thereof is fitted in the large diameter bore 3 between the 
inlet fitting 7 and the guide member 13. An annular seat member 19 made of 
an elastic material is fitted in the inside opening of the supporting 
member 18 with the annular flange thereof engaging an annular groove 
formed in the inner circumference of the supporting member 18. A valve 
seat 20 for receiving the ball 14 is provided in a portion of the seat 
member 19 fitted in the inside opening of the brim of the guide member 13. 
The retainer 16, the guide member 13 and the supporting member 18 holding 
the seat member 19 are positioned with respect to the axial direction 
between the inner end of the large diameter bore 3 and the inner end of 
the inlet fitting 7. 
A passage 22 is formed above the large diameter bore 3 to connect a cavity 
defined by the outer circumference of the inlet fitting 7 and the inner 
circumference of the large diameter bore 3 to a medium diameter section 23 
of the vertical bore 4. The inlet passage 10 communicates with the outlet 
passage 12 by means of a first or bypass air passage 21 including the 
inside hole of the seat member 19, the interior of the guide member 13, 
the through hole 6 and the large diameter section 5 of the vertical bore 
4, and also by means of a second passage 26 including the cavity, the 
passage 22, the medium diameter section 23 of the vertical bore 4 and the 
large diameter section 5 of the same. The ball 14 and the valve seat 20 
constitute a holding valve 27 for intercepting the flow of air through the 
first passage 21 from the outlet port 11 toward the inlet port 9. A check 
valve 28 is provided within the large diameter section 5 to intercept the 
flow of air through the second passage 26 from the outlet port 11 toward 
the inlet port 9. The holding valve 27 and the check valve 28 constitute a 
valve device for intercepting the flow of air from the outlet port 11 
toward the inlet port 9. The check valve 28 comprises a disk-shaped valve 
element 30 having projections projecting radially outwardly from the 
circumference thereof, a valve seat 32 provided on the upper end of the 
large diameter section 5, and a comparatively weak valve spring 31 biasing 
the valve element 30 toward the valve seat 32. 
A cover 33 is airtightly attached to the upper surface of the main body 2 
so as to cover an upper large diameter section 25 of the vertical bore 4, 
and a stepped cylindrical hole 34 is formed coaxially with the vertical 
bore 4 in the cover 33. The cylindrical hole 34 has a large diameter 
section 35 and a small diameter section 36. The large diameter section 35 
of the cylindrical hole 34 is contiguous to the upper large diameter 
section 25 of the vertical bore 4. The inside diameter of the upper large 
diameter section 25 of the vertical bore 4 is smaller than the inside 
diameter of the large diameter section 35 and is greater than the inside 
diameter of the small diameter section 36. 
A movable member 37 having a large section 38 and a small section 39 
provided with sealing rings in the respective circumferences thereof is 
fitted in the cylindrical hole 34 so that the large section 38 and the 
small section 39 are axially movable in the large diameter section 35 and 
the small diameter section 36 of the cylindrical hole 34, respectively. 
The axial movement of the large section 38 is limited by a shoulder 40 
defined between the large diameter section 35 and the small diameter 
section 36, and a shoulder 41 defined between the large diameter section 
35 and the vertical bore 4. A stem 42 is formed integrally with the 
movable member 37. The stem 42 extends slidably through a small diameter 
section 24 provided with a sealing ring via the medium diameter section 23 
into the lower large diameter section 5 of the vertical bore 4. The lower 
end of the stem 42 is able to engage the valve element 30 of the check 
valve 28. The length of the stem 42 is determined so that the stem 42 
pushes the valve element 30 away from the valve seat 32 to open the check 
valve 28 when the movable member 37 is at a first position wherethe lower 
end of the large section 38 is in contact with the shoulder 41 and so that 
the stem 42 allows the valve spring 31 to seat the valve element 30 on the 
valve seat 32 so that the check valve 28 is able to function normally 
according to a differential pressure. 
A control pressure chamber 43 is defined by the lower end of the large 
section 38 of the movable member 37 and the upper large diameter section 
25 of the vertical bore 4. The control pressure chamber 43 communicates 
with a control port 44 formed in the right side surface, as viewed in FIG. 
1, of the main body 2. An outlet pressure chamber 45 is defined by the 
upper end of the small section 39 of the movable member 37 and the small 
diameter section 36. The outlet pressure chamber 45 communicates with a 
part of the holding valve 27 on the side of the outlet port 11 by means of 
a passage 46 (indicated by broken lines in FIG. 1) formed in the cover 33, 
a pipe 47 attached to the main body 2, a bore 48 formed in the main body 
2, and a recess formed in the right end of the guide member 13 opposite to 
the bore 48. A spring 50 is provided in the small diameter section 36 to 
apply a predetermined pressure to the small section 39 of the movable 
member 37 so as to bias the movable member 37 downward, namely, toward the 
first position, against the frictional resistance of the sealing rings 
provided on the large section 38 and the small section 39 of the movable 
member 37, and the sealing ring provided in the annular groove of the 
small diameter section 24 of the vertical bore 4. A chamber defined by the 
shoulder 40 and the large section 38 of the movable member 37 always 
communicates with the atmosphere by means of a small hole 52 formed in the 
side wall of the cover 33. 
The cover 33 having the cylindrical hole 34, the movable member 37 having 
the stem 42, the control pressure chamber 43, the outlet pressure chamber 
45 and the spring 50 constitute a control unit 51 for controlling the 
check valve 28. The braking force holding device 1 having the control unit 
51 is mounted on the chassis of the vehicle with the side having the 
control port 44, namely, the right-hand side of the main body 2 as viewed 
in FIG. 1, on the front side of the vehicle. 
The inlet port 9 is connected to a main brake control device, not shown, 
such as a brake valve which is connected to a compressed air reservoir and 
discharges compressed air when operated by the driver or a relay valve 
which receives a pilot pressure from a brake valve and discharges 
compressed air under a pressure corresponding to the pilot pressure. The 
outlet port 11 is connected to a driven device, not shown, such as a brake 
actuator, a brake chamber or an air over hydraulic booster which is driven 
by compressed air to actuate the brakes of the vehicle, or a relay valve 
which receives a pilot pressure from the main brake control device and 
supplies a pressure corresponding to the pilot pressure to the brake 
actuator. The control port 44 is connected to the actuator of a clutch 
actuating device, not shown, such as a clutch booster, which is driven by 
compressed air to operate the clutch disposed between the engine and the 
transmission of the vehicle. The lower end of the large section 38 of the 
movable member 37 of the control unit 51 receives a pressure produced by 
the clutch booster, while the upper end of the small section 39 of the 
movable member 37 of the same receives a pressure applied to the driven 
device through the outlet port 11. 
The operation of the braking force holding device will be described 
hereinafter. 
The braking force holding device 1 is in a state shown in FIG. 1 while the 
vehicle is running on a flat road and neither the brake nor the clutch is 
operated. The ball is separated from the valve seat 20 and is in abutment 
with the retainer 16 to open the holding valve 27. Only the resilience of 
the spring 50 is acting on the movable member 37 of the control unit 51 to 
hold the movable member 37 at the first position where the large section 
38 is seated on the shoulder 41 and the stem 42 positions the valve 
element 30 of the check valve 28 away from the valve seat 32 to open the 
check valve 28, and the inlet port 9 communicates with the outlet port 11 
by means of the first passage 21 and the second passage 26. 
When the vehicle comes to a hill and begins to climb the hill, the braking 
force holding device 1 is inclined right-hand side up, as viewed in FIG. 
1. Consequently, the ball 14 moves toward the valve seat 20 and is seated 
on the valve seat 20 to close the holding valve 27. When the brakes are 
applied in this state to stop the vehicle, compressed air discharged from 
the brake valve is supplied through the inlet port 9, the inlet passage 
10, the second passage 26 and the outlet port 11 to the relay valve to 
actuate the brake actuator. A portion of the compressed air thus supplied 
moves the ball 14 away from the valve seat 20 and flows through the first 
passage 21 toward the outlet port 11. However, upon the movement of the 
ball 14 away from the valve seat 20, the pressure on opposite sides of the 
ball 14 is equalized, and hence the ball 14 moves toward the valve seat 20 
and is seated again on the valve seat 20 to close the holding valve 27 
again. Compressed air supplied through passage 26, passage 6, the inlet 
port 9 flows through the bore 48 and the passage 46 into the outlet 
pressure chamber 45 of the control unit 51 to apply a pressure to the 
upper end of the small section 39 of the movable member 37. 
When the driver steps on the clutch pedal to disengage the clutch after the 
vehicle has been braked and has sufficiently been decelerated, compressed 
air supplied to the clutch booster flows through the control port 44 into 
the control pressure chamber "43" to apply a pressure to the lower end of 
the large section 38 of the movable member 37. As the clutch pedal is 
depressed further, the pressure within the control pressure chamber 43 
increases. When the force acting on the lower end of the large section 38 
of the movable member 37 exceeds the force pressing the movable member 37 
toward the first position, the movable member 37 is moved upward. As the 
movable member 37 is moved upward, the valve element 30 of the check valve 
28, which has been separated from the valve seat 32 by the stem 42 of the 
movable member 37, is moved toward the valve seat 32 by the valve spring 
31 and, finally, the valve element 30 is seated on the valve seat 32 to 
close the check valve 28. When the movable member 37 is moved to the 
second position where the upper end of the large section of the movable 
member 37 is brought into contact with the shoulder 40, the lower end of 
the stem 42 is separated from the valve element 30 of the check valve 28 
to allow the check valve 28 to operate normally for intercepting,only the 
flow of air from the outlet port 11 through the passage 26 toward the 
inlet port 9. 
After the clutch has been entirely disengaged and the vehicle has been 
stopped, the holding valve 27 and the check valve 28 are closed, so that 
the flow of air from the outlet port 11 through the first passage 21 and 
the second passage 26 toward the inlet port 9 is intercepted. 
Consequently, the pressure supplied to the relay valve by the compressed 
air supplied thereto through the outlet port 11 is maintained to hold the 
braking force as long as the clutch is held disengaged, even when the 
compressed air supplied to the inlet port 9 is discharged by the brake 
valve by releasing the brake pedal. Thus, the vehicle is kept in a braked 
state. 
Since the outlet pressure chamber 45 of the control unit 51 always 
communicates with the outlet port 11 by means of the outlet passage 12, 
the through hole 6, the recess 49, the bore 48 and the passage 46 
regardless of the condition of the holding valve 27 and the check valve 
28, the pressure of the compressed air held in the relay valve works 
continuously on the upper end of the small section 39 of the movable 
member 37 while the vehicle is stopped. However, since the pressure 
applied to the lower end of the large section 38 of the movable member 37 
by the clutch booster when the clutch was disengaged remains effective and 
the force acting on the lower end of the large section 38 of the movable 
member 37 is greater than the force acting on the upper end of the small 
section 39 of the movable member 37, the movable member 37 is restrained 
at the second position. 
When the transmission is shifted to a desired gear, and the clutch is 
engaged gradually as the accelerator is operated to increase the engine 
speed to start the vehicle, the pressure prevailing in the control 
pressure chamber 43 is reduced gradually as the clutch is engaged 
progressively. Consequently, the force restraining the movable member at 
the second position against the resilience of the spring 50 and the force 
produced by the pressure prevailing within the outlet pressure chamber 45 
corresponding to the pressure holding the braking force decreases 
gradually and, finally, the force retaining the movable member 37 at the 
second position decreases below the force pressing the movable member 37 
toward the first position. Then, the movable member 37 is moved down 
toward the first position, and then the lower end of the stem 42 is 
brought into contact with the valve element 30 of the check valve 28 to 
separate the valve element 30 from the valve seat 32, so that the check 
valve 28 is opened. Consequently, the compressed air held in the relay 
valve is allowed to flow through a gap between the valve element 30 and 
the valve seat 32, the second passage 26 and the inlet port 9, and is 
discharged gradually from the brake valve, and thus the pressure held in 
the brake actuator is reduced accordingly. When the check valve 28 is 
opened, the compressed air held within the outlet pressure chamber 45 of 
the control unit 51 is discharged through the passage 46, the bore 48, the 
recess 49, the through hole 6, and the inlet port 9, and thus, the 
pressure prevailing within the outlet pressure chamber 45 is reduced 
gradually. However, the movable member 37 is moved down by the resilience 
of the spring 50 to the first position until the lower end of the large 
section 38 abuts against the shoulder 41, whereby the check valve 28 is 
held open by the stem 42. Thus, as the clutch is engaged progressively to 
transmit the driving force increasingly to the driving wheels, the braking 
force is reduced accordingly. 
The pressure to be held on the side of the outlet port 11 to hold the 
vehicle stationary on an upward hill is proportionally dependent on the 
grade of the hill and the weight of the vehicle. However, the higher the 
braking pressure held to hold the vehicle on the hill, the earlier the 
opening of the check valve as the clutch is engaged progressively to start 
the vehicle. That is, the higher the pressure held on the side of the 
outlet port 11 and, hence, the higher the pressure prevailing within the 
outlet pressure chamber 45 of the control unit 51, the greater is the 
force acting on the movable member 37 to urge the same toward the first 
position for opening the check valve 28. Therefore, the difference between 
the force urging the movable member 37 toward the first position and the 
force corresponding to the pressure of the compressed air supplied to the 
control pressure chamber 43 when the clutch was disengaged to restrain the 
movable member 37 at the second position is small while the braking 
pressure is held. Accordingly, the force urging the movable member 37 
toward the first position exceeds the force urging the movable member 37 
toward the second position when the pressure within the control pressure 
chamber 43 is reduced by a small degree as the clutch is engaged 
progressively, and hence the movable member 37 starts moving toward the 
first position at an early timing relative to the degree of engagement of 
the clutch. Consequently, the check valve 28 is opened by the stem 42 at 
an advanced timing where the degree of engagement of the clutch is small. 
Thus, the timing of the opening of the check valve 28 relative to the 
degree of engagement of the clutch is controlled corresponding to the 
magnitude of the pressure held on the side of the outlet port 11. 
Therefore, the braking pressure is reduced to an extent for providing a 
braking force necessary only for checking the backward movement of the 
vehicle when the clutch is in slipping engagement, even when a high 
pressure is held on the side of the outlet port 11. Accordingly, the 
vehicle starts climbing the hill without entailing adverse influence on 
the clutch and the brakes. 
A braking force holding device, in a second embodiment according to the 
present invention will be described hereinafter with reference to FIG. 2. 
The braking force holding device 101 has a main body 102 having a passage 
160 interconnecting an inlet port 109 and an outlet port 111. Only a 
holding valve 127 having a ball 114 capable of moving toward a valve seat 
120 when the vehicle comes to a hill and faces upwardly is provided in the 
passage 160. A control unit 151 for controlling the holding valve 127 is 
disposed coaxially with the passage 160. The driving force holding device 
101 is mounted on the vehicle with the right-hand side, as viewed in FIG. 
2, of the main body 102 provided with the outlet port 111 disposed on the 
front side of the vehicle. The control unit 151 has a construction similar 
to the control unit 51 of the first embodiment. Compressed air supplied 
through a control port 144 into a control pressure chamber by a clutch 
booster applies a pressure to a movable member 137 having a stem 142 
capable of engaging the ball 114 so as to urge the movable member 137 in a 
direction to close the holding valve 127 while compressed air supplied 
from the outlet port 111 into an outlet pressure chamber 145 applies a 
pressure to the movable member 137 so as to urge the movable member 137 in 
a direction to open the holding valve 127. The movable member 137 moves in 
either direction depending on the pressure, difference between the control 
pressure chamber 143 and the outlet pressure chamber 145. 
The second embodiment is different from the first embodiment in that the 
opening of the holding valve 127 is controlled so that the time at which 
the braking force is relieved is advanced relative to the degree of 
engagement of the clutch with the increase of the pressure held on the 
side of the outlet port 111. The operation of the second embodiment is 
substantially the same as that of the first embodiment. Therefore the 
description of the operation will be omitted. 
Thus, according to the present invention, the pressure of compressed air 
held in the driven device on the side of the outlet port 11, 111 is 
applied to the movable member 37, 137 of the control unit 51, 151 so as to 
urge the movable member 37, 137 toward the first position to open the 
check valve 28 or the holding valve 127 while the pressure of compressed 
air supplied to the clutch booster is applied to the movable member 37, 
137 of the control unit 51, 151 so as to urge the movable member 37, 137 
toward the second position to enable the check valve 28 to operate 
normally or to close the holding valve 127, and the movable member 37, 137 
is shifted between two positions corresponding to the first position and 
the second position depending on the difference between the opposite 
forces produced by the respective pressures for controlling the check 
valve 28 or the holding valve 127. Accordingly, the braking force is 
maintained as long as the clutch remains disengaged and the braking 
pressure is relieved as the clutch is engaged progressively. Consequently, 
the vehicle can easily be started on an upward hill without requiring the 
operation of the hand brake in combination with the dexterous manipulation 
of the clutch and the accelerator. Furthermore, since the higher the 
braking pressure held on the side of the outlet port 11, 111, the earlier 
the check valve 28 opens or the holding valve 127 opens for relieving the 
braking pressure relative to the degree of frictional engagement of the 
clutch. The adverse action of an excessive braking force against the 
driving force transmitted to the driving wheels, the excessive slip of the 
clutch, the drag of the brakes and engine stop in starting the vehicle are 
prevented. 
Additionally, since the compressed air supplied to the clutch booster of 
the clutch actuating device is also supplied through the control port 44, 
144 into the control pressure chamber 43, 143, and the control unit 51, 
151 utilizes the pressure of the compressed air supplied into the control 
pressure chamber 43, 143 for controlling the check valve 28 or the holding 
valve 127, the braking force holding device according to the present 
invention comprises only pneumatic components. Accordingly, conditions of 
pressure tightness and strength of the main body 2, 102 and the components 
of the control unit 51, 151 of the braking force holding device according 
to the present invention are relaxed as compared with the conventional 
braking force holding device employing liquid pressure which produces a 
force greater than that produced by compressed air, produced by the clutch 
actuating device. Thus, the present invention improves the productivity of 
the production line for producing the braking force holding device, and 
enhances the reliability of the braking force holding device. 
Although the present invention has been described with reference to the two 
preferred embodiments thereof by way of example, the invention is not 
limited thereto and many variations and changes are possible in the 
invention without departing from the scope thereof. 
For example, the movable member 37, 137 having the shape of a piston 
slidably fitted in the cylindrical bore of the control nit .may be 
substituted by a diaphragm having an integral operating member capable of 
engaging the valve element of the check valve 28 or the holding valve 127, 
and disposed so as to receive the pressure on the side of the control port 
and the pressure on the side of the outlet port on the opposite sides 
thereof, respectively. When such a diaphragm is employed as the movable 
member, it is possible to omit the spring for urging the movable member in 
a direction to open the check valve or the holding valve. It is also 
possible to control the valve device, namely, the check valve or the 
holding valve, by disposing the control unit so that the axis thereof 
intersects the axis of the valve device and by transmitting the 
displacement of the, movable member through another movable member having 
a surface inclined to the valve element of the valve device, instead of 
disposing the control unit and the valve device so that the movable member 
of the control unit and the valve element of the valve device are coaxial. 
Furthermore, it is also possible to control the valve device by disposing 
the control unit on the side of the outlet port of the valve device, 
instead of disposing the same on the side of the inlet port of the valve 
device. Still further, it is possible to employ a valve device comprising, 
in a parallel arrangement, an electromagnetic valve capable of isolating 
the inlet port and the outlet port from each other upon the reception of 
an electric signal provided by means for detecting the grade of the road 
and the running speed of the vehicle, and a check valve for intercepting 
the flow of air from the outlet port toward the inlet port, and to control 
the timing of the opening of the check valve. 
Although the invention has been described in its preferred forms with a 
certain degree of particularity, it is to be understood that many 
variations and changes are possible in the invention without departing 
from the scope and spirit thereof.