TM 9-2330-294-14
brake chambers. Brake air pressure is equal to the
bubble in 3 seconds. If excess leakage is found,
signal pressure in the service air line. When the
replace emergency relay valve. Refer to paragraph 4-
towing vehicle brake pedal is released, brake air
42 for emplacement.
chamber air is vented through the relay valve, thus
g. Service Brake Manual Release Valve Operation
releasing the semitrailer brakes.
and Test.
(b ) If the semitrailer breaks away from the
towing vehicle, the pressure loss in the emergency
handle has two positionsin and out. The handle
air line is sensed by the relay valve. The relay valve
pops out automatically when the semitrailer
reacts by sending full pressure semitrailer reservoir
emergency air line is pressurized, and this is its
air to the service brakes to stop the semitrailer. This
normal position. In this position the semitrailer
emergency operation also occurs when the
emergency air line is connected to the semitrailer
emergency relay valves. When this emergency air
semitrailer is merely uncoupled from the towing
line is depressurized the emergency relay valves
vehicle or when the towing vehicle parking brake is
apply the semitrailer service brakes. The service
applied.
(4) Operating tests.
brakes are applied under three circumstances:
(a ) The parking brake is applied in the
towing vehicle, apply brake treadle in towing
towing vehicle.
(b ) The semitrailer is parked and the air lines
vehicle, and check to be sure that brakes on all
disconnected from the towing vehicle.
semitrailer wheels engage.
(c ) The semitrailer breaks away from its
(b ) Release brakes, and check to be sure that
towing vehicle.
each brake dieengages promptly. When towing
(d ) The service brake manual release valve is
vehicle brake treadle is released, brake chamber air
used to release the service brakes once they have
should vent through the exhaust port of the relay
been applied by any of the above actions. The release
valve. If this venting does not occur, the relay valve
valve handle is pushed IN. This action isolates the
is defective.
trailer emergency air line and uses stored system air
(c ) With brake system fully pressurized
to pressurize the emergency air port of the relay
close shutoff cock in emergency line on towing
valves thus releasing the service brakes.
vehicle, and disconnect air hose coupling tagged
(2) Test.
EMERGENCY. Check to be sure semitrailer brakes
(a ) Pressurize semitrailer air system.
apply automatically.
(b ) Disconnect intervehicular emergency air
(d ) Connect air hose coupling tagged
line.
EMERGENCY, open shutoff cock on towing
(c ) Verify valve handle is in the OUT
vehicle, and check to be sure brakes release
position (brakes applied).
automatically.
CAUTION
(e ) If brakes malfunction, assure that the
Do not use pliers to loosen fitting. Use only
emergency relay valve is making contact with a
proper wrench. Unscrew fitting only a few
pressure signal in the service air line when the brakes
threads.
are applied. (Disconnect service air line and depress
(d ) Loosen fitting on air line from manual
brake pedal to witness the escape of pressured air).
release valve to emergency relay valves (fig. 4-27 and
Assure that the relay valve is in contact with the air
4-28). No air should escape from the fitting unless
pressure available in the emergency and reservoir air
the release valve handle is pushed IN.
lines. (Check system air pressure with tire air gage
(e ) Push handle of manual release valve IN.
and assure that no kinks are present in the air lines. )
Air should escape forcefully from the loosened fit-
If required air pressure is applied to relay valve port,
ting.
but the brakes fail to operate correctly, replace the
(f ) Replace valve if air escapes when valve
emergency relay valve (para 4-42).
handle is OUT or if air does not escape when valve is
(5) Leakage test.
IN. See paragraph 4-45 for replacement procedure.
(a ) With air brake system connected to
h. Fail-Safe Brake Manual Release Valve
towing vehicle air supply, depress and hold cab
Operation and Test.
brake pedal. Apply soapy water to flanges of relay
valve which hold diaphragm, and to service air line
mechanically identical to the service brake manual
couplings. No leakage is permitted. Tighten nuts on
release valve. When the semitrailer air system is
flanges, and tighten coupling as required.
pressurized the control knob handle on this valve
(b ) Repressurize vehicle emergency air line,
pops OUT to its normal position. In this position the
and coat exhaust ports of emergency relay valves
fail-safe brake air chambers are pressurized by
with soapy water. Leakage must not exceed a 1-inch
