TM 9-2330-271-14&P
Figure 4-22. Brake system components.
d. Compressed Air Supply.
(3) Releasing chassis brakes. When the brake
To produce and
maintain a constant supply of compressed air to control
pedal is released, a drop in pressure causes the relay
and actuate the hydraulic brake system, the towing
valve to release the compressed air from the chassis
service brake system, and springs pull the brake shoes
reservoirs, a governor for controlling the air pressure, an
away from the drums. The extent of brake release is in
air gage and a safety valve. Air lines, air hose, and shut-
f.
off valves convey compressed air to the trailer service
brake control system.
and 4-22) directly controls the service brakes. This valve
e. Functioning of Service Brakes.
controls the flow of air to and from the air reservoir and
(1) Semitrailer chassis air reservoir. When the
automatically applies the brakes if the trailer breaks away
air hose couplings are connected between the towing
from the towing vehicle or if there is a serious leak in the
vehicle and the chassis, air shutoff valves on the towing
emergency air line. Provisions are made for connections
vehicle are opened. Air flows through the air line, air
to the emergency air line, the service air line, air
filter, and relay valve to fill the air reservoir on the
reservoir, and the brake air chambers and to exhaust
chassis. Air pressure is built up to equal the pressure in
compressed air that has been used.
g. Hydraulic Master Cylinder.
The two
the towing vehicle's system.
(2) Applying the chassis brakes.
When
hydraulic master cylinders (fig. 4-23) are attached to the
pressure is applied to the brake pedal, air pressure is
two brake air chambers and the assemblies are
directed to the relay valve.
This valve releases
compressed air from the air reservoir to the brake air
chamber attached to the hydraulic master cylinder. The
air chamber actuates the hydraulic master cylinder to
develop hydraulic pressure which moves the wheel
cylinder piston in the wheel brake mechanism. These
pistons force the lining of the brake shoe against the
brake drum.
4-38