TM-9-2320-272-10
TRUCK, 5-TON, 6X6, M939, M939A1, AND M939A2 SERIES TRUCKS (DIESEL)
TECHNICAL MANUAL; OPERATOR'S MANUAL
SEPTEMBER 2004
CHANGE 2
TM-9-2320-272-10 - Page 84 of 481
TM 9-2320-272-10
1-20. COMPRESSED AIR AND BRAKE SYSTEM OPERATION (Contd)
E
PEDAL INTO VALVE
– Allows air pressure from primary constant pressure
system to flow into primary signal system when operator depresses brake
pedal.
DOUBLECHECK VALVE #1
–
Serves two functions:
• Allows system to receive signal pressure from either pedal valve or, when
towed, from brake system of towing vehicle.
• Serves as a tee between front and rear primary signal lines.
• Removed when ABS kit is installed and replaced with cross tee.
FRONT SERVICE COUPLING
– When vehicle is being towed, coupling is
connected to towing vehicle so that the brake systems of the two vehicles
work together.
ONE-WAY CHECK VALVE
– Allows air pressure to flow into primary
reservoir but prevents it from coming out if constant pressure system fails or
engine stalls.
PRIMARY AIR RESERVOIR
– Stores enough air pressure so the operator
can make five normal brake applications before running out of air if constant
pressure fails or engine stalls.
DOUBLECHECK VALVE #2
– Allows either primary or secondary signal
air pressure to activate stoplight switch while keeping the two systems
separate.
STOPLIGHT SWITCH
–
As the brake pedal is depressed, switch receives
an air pressure signal which closes electric contacts turning on stoplight.
REAR SERVICE COUPLING
– When towing another vehicle, coupling is
connected to towed vehicle so that the brake system of the two vehicles work
together.
INTERMEDIATE FRONT BRAKE CHAMBERS
–
Converts air pressure
to mechanical force which applies intermediate rear service brake.
PRIMARY RESERVOIR DRAINVALVE
–
Provides a drain for moisture
and air from primary air reservoir.
NOTE
On vehicles equipped with ABS, the
rear relay valve operates the rear-
rear right hand brake chamber and
intermediate rear right hand brake
chamber.
REAR RELAY VALVE
–
Serves three functions:
• Boosts signal air pressure to rear brake chambers. Air signal from brake
pedal opens valve to route constant air pressure to rear brake chambers.
• Regulates signal air pressure from brake pedal to rear brake chambers so
operator has control over amount of braking. Regulates amount of constant
air pressure going to brake chambers as the operator depresses the brake
pedal.
• Releases air pressure in rear brake chamber directly to vent system when
brake pedal is released.
REAR FRONT BRAKE CHAMBERS
– Converts air pressure to
mechanical force which applies rear service brakes.
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