mounted. A return spring forces the power piston to the
released position when the brake pedal is released.
The SLAVE CYLINDER consists of a
cylindrical housing in which a small diameter hydraulic
piston operates. The outlet cap houses a residual check
valve and a ball-check valve is located in the hydraulic
The CONTROL VALVE consists of two
poppets operating within a housing and actuated by a
hydraulic relay piston and a reactionary-type
diaphragm. An air control line connects the control
valve to the compressed air cylinder.
the relay piston in the control valve. As hydraulic
pressure builds, the relay piston moves the control
valve diaphragm forward, closing the atmospheric
poppet and slightly opening the air pressure poppet.
Air under pressure then passes through the air control
line forcing the power piston in the air cylinder
forward until the air pressure on the diaphragm, in
combination with spring pressure, allows the air
poppet to close. The degree of brake application is
determined by the amount of compressed air trapped in
the power cylinder when brake pedal movement is
stopped. Unless more pressure is applied or the brake
pedal is released, the brakes will remain in the partially
The air-over-hydraulic cylinder consists of an air
cylinder and hydraulic cylinder in tandem, each fitted
with a piston with a common piston rod between. The
air piston is of greater diameter than the hydraulic
piston. This difference in areas of the two pistons gives
a resultant hydraulic pressure much greater than the air
pressure admitted to the air cylinder. Automatic
valves. actuated by fluid pressure from the master
cylinder, control the air admitted to the air cylinder.
Thus fluid pressure in the brake lines is always in direct
ratio to foot pressure on the brake pedal.
View B of figure 7-61 shows the effect of applying
high brake pedal pressure. Under this condition, the air
pressure poppet is held open, allowing a full volume of
compressed air to enter the air cylinder and cause full
As in a conventional hydraulic brake system the
residual check valve maintains a small amount of
pressure in the hydraulic system when the brakes are
released. This prevents the cups in the wheel cylinders
from collapsing and leaking.
REVIEW 3 QUESTIONS
Figure 7-61 shows the air-hydraulic power
cylinder in the released position. Views A and B show
the position of the valves and slave cylinder during
light and heavy brake pedal application.
What are the three essential components of an
Q2. In what component of the air-hydraulic-power
cylinder is the residual check valve located?
When the brakes are applied, as shown in view A
What action actuates the automatic valves that
of figure 7-61. pressure is transmitted by the brake
control the air pressure admitted to the air
fluid to the hydraulic piston in the slave cylinder and