TM 9-2330-294-14
safe units will automatically unlock and release the
brakes. When the semitrailer is parked and un-
The service brakes are actuatd by the semitrailer air
coupled, the fail-safe units will gradually become
system which is maintained at 90-120 psi air
applied as the air pressure will eventually drop below
pressure by the towing vehicle. The controls are
65 psi which is required to hold back the fail-safe
located in, and are a part of, the towing vehicle. The
springs. When parked, or when the air system is
service brakes are activated when the towing vehicle
disabled, the semitrailer can be moved without
brake pedal is depressed and an air pressure signal is
charging the whole air system by utilizing air from
transmitted through the semitrailer service air line
the fail-safe release air tank (para 2-4b). If the fail-
to the emergency relay valves. The emergency relay
safe release system is inoperative, the fail-safe unit
valves then operate automatically to route reservoir
springs must be compressed manually, as follows,
air through the service brake air supply lines to the
before moving the semitrailer:
air chambers to apply the brakes. Conversely, when
WARNING
the towing vehicle brake pedal is released, the
Block wheels to prevent semitrailer from
pressure signal ceases and the emergency relay
rolling free when fail-safe brakes are
valves vent air pressure and release the brakes. The
released.
semitrailer service brakes will also activate
automatically if the semitrailer breaks away from the
CAUTION
towing vehicle. When the towing vehicle parking
Stripping of the release bolt threads may
brake is applied, the semitrailer emergency air line is
result if the release bolt is forced beyond its
exhausted and the emergency relay valves route
natural strep when being turned in either
captive air to apply the service brakes. When the
direction.
service brakes are applied as a result of a severe leak
Turn release bolt approximately 25 full turns clock-
or uncoupling from the towing vehicle, the brakes
wise on each fail-safe unit. The 25 full turns should
can be released by pushing the service brake release
provide sufficient travel of the spring loaded piston
knob on the semitrailer left side. Refer to paragraph
within the fail-safe unit to release the brake, After
2-4 for operating instructions of the brake release
moving semitrailer, block wheels and release the fail-
controls.
safe power springs by turning release bolt ap-
proximately 25 full turns counterclockwise on each
fail-safe unit. The 25 full turns should provide
One brake air chamber on each axle end is equipped
sufficient travel of the springs loaded piston within
with a fail-safe unit. These units will automatically
the fail-safe unit to apply the brake. Refer to figure
apply the semitrailer brakes in the event air pressure
4-25 and 4-26 for a schematic representation of the
in the reservoir air line falls below 65 psi. When air
fail-safe unit.
pressure has been restored to 65 psi or more the fail-
