TM-9-2320-272-24-1
TRUCK, 5-TON, 6X6, M939, M939A1, AND M939A2 SERIES TRUCKS (DIESEL)
TECHNICAL MANUAL; UNIT, DIRECT SUPPORT, AND GENERAL SUPPORT MAINTENANCE MANUAL; VOLUME 1 OF 4
JUNE 1998
TM-9-2320-272-24-1 - Page 71 of 1015
TM 9-2320-272-24-1
1-23. COMPRESSED AIR AND BRAKE SYSTEM OPERATION (Contd)
Key Item and Function
A
B
C
D
E
F
G
H
I
J
K
L
LOW AIR PRESSURE SWITCH
- Activates warning buzzer and warning lights when air pressure
goes below 60 psi (414 kPa).
PEDAL VALVE
- Allows air pressure from secondary constant pressure system to flow into
secondary signal system when operator depresses brake pedal.
SECONDARY AIR PRESSURE GAUGE
- Indicates amount of air pressure in secondary system.
ONE-WAY CHECKVALVE
-Allows air pressure to flow into secondary reservoir, but prevents it
from coming out if constant pressure system fails or engine stalls.
SECONDARY 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.
INTERMEDIATE REAR BRAKE CHAMBERS
- Converts air pressure to mechanical force which
applies intermediate rear service brake.
l
Allows system to receive signal pressure from either pedal valve or, when towed, from brake
system of towing vehicle.
DOUBLECHECK VALVE NO. 1
- Serves two functions:
l
Serves as a tee between front and rear primary signal lines.
SECONDARY AIR RESERVOIR DRAINVALVE
- Provides a drain for moisture and air from
secondary air reservoir.
FRONT RELAY VALVE
- Boosts signal air to rear brake chambers; regulates air pressure to rear
brake chambers so operator has control over amount of braking; and releases air pressure to rear
brake chambers directly to vent when brake pedal is released.
STOPLIGHT SWITCH
- As the brake pedal is depressed, switch receives an air pressure signal at
electrical contacts which close to activate circuits to taillights.
DOUBLECHECK VALVE NO. 2
- Allows either primary or secondary signal air pressure to
activate stoplight switch while keeping the two systems separate.
REAR-REAR BRAKE CHAMBERS
- Converts air pressure to mechanical force which applies rear-
rear brakes.
1-61
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