TM 9-2330-388-14
f.
Fail-Safe Units, Fig. 4-43. The fail-safe units
(2) When there is enough reserve air in front
reservoir, the spring brakes can be released opening a
are spring chambers which are assembled piggyback on
valve on the roadside of the semitrailer.
the brake air chambers. One is installed on the air
chambers at each of the four corners of the tandem axle
d.
Service Brake. When the towing vehicle is
assembly. The fail-safe unit provides parking and
attached, the foot brake operates the brakes on both the
emergency braking.
semitrailer and the towing vehicle.
(1) Air pressure of approximately 65 psi is
(1) Applying the brake pedal delivers through
the service line to the emergency relay valve.
(2) In response to the service signal, the
emergency relay valve releases air form the rear
reservoir to the brake chambers. Air pressure causes
the camshaft in each brake assembly to rotate and force
the brake shoes against the brake drum.
(3) Releasing the brake pedal stops the service
signal to the emergency relay valve. the emergency
relay valve closes the supply from the reservoir and
releases air from the brake chambers.
e.
Emergency Relay Valve, Fig. 4-42. This valve
increases the speed at which brakes apply and release.
Figure 4-42. Emergency Relay Valve
The relay valve is attached to the rear reservoir above
the forward axle.
needed to release the fail-safe units.
(1) The emergency relay valve releases air
(2) Loss of air pressure will release the heavy
from the rear (primary) reservoir to all brake chambers in
spring and push the piston against the diaphragm plate
response to the service signal from the towing vehicle.
of the air chamber. This force is applied through the
camshaft to the brake shoes.
(2) The emergency relay valve will also cause
the fail-safe brakes to apply automatically if the semi-
g.
Slack Adjusters. These convert linear motion of
trailer breaks away from the towing vehicle.
the brake chamber to rotating motion of the camshaft.
Figure 4-43. Brake Actuator Assembly (M1098)
4-107
