1
2
4
5
3
6
12
11
7
10 9
8
1
HYDRAULIC LINE
PRESSURE LINE
ASf02045
APPLY LINE
Wheel cylinder connectors
Control valve
Master brake cylinder
1.
5.
9.
Tank drain valve
Control air line
System governor
2.
6.
10.
System safety valve
Air compressor
Hydraulic line
3.
7.
11.
Compressed air line
Air chamber
Air reservoir
4.
8.
12.
Figure 2-45.--Air-over-hydraulic brake system.
The piston and piston rod of the air chamber are
Brake application is relative to the pressure
applied by the operator on the brake pedal. The brakes
connected to the push rod of the master cylinder. When
of the vehicle may be partially released at any time by
the brakes are applied, the air control valve allows air
slightly relieving pressure on the pedal or entirely
under pressure to enter the air pressure port of the air
released by removing all pressure from the pedal. One
chamber. This pressure forces the diaphragm to move
of the ports of the air control valve is an exhaust port,
the piston and piston rod in the air chamber and
which releases air from the air chamber of the master
compresses the return spring. This movement, in turn,
cylinder during the release action.
moves the push rod and piston in the master cylinder,
forcing fluid pressure to the wheel cylinders. When the
The master brake cylinder assembly is the point in
brakes are released, the air control valve stops the flow
this system where the pneumatic system and hydraulic
of air to the air chamber and at the same time opens the
system join. Figure 2-46 shows a partial cutaway view
exhaust port. This allows the air to flow out of the
of the assembly. Only the air chamber is shown cut
chamber, through the line, and out the exhaust port of
away because the design and operation of the master
the control valve. The return spring returns the piston
cylinder part of the assembly are similar to any other
and piston rod to the release position. This movement
hydraulic master brake cylinder.
RETURN SPRING
DIAPHRAGM
BREATHER PORT
PISTON
MASTER CYLINDER
AIR PRESSURE
PORT
PISTON
ROD
SLEEVE
ASf02046
Figure 2-46.--Master cylinder assembly--air-over-hydraulic brake system.
2-37
