TM-9-2320-272-23-1

TRUCK, 5-TON, 6X6, M939, M939A1, AND M939A2 SERIES TRUCKS (DIESEL)

TECHNICAL MANUAL; FIELD MAINTENANCE MANUAL; VOLUME 1 OF 5

SEPTEMBER 2012

  TM-9-2320-272-23-1 - Page 245 of 1313

SECONDARY SERVICE AIRBRAKE SYSTEM OPERATION
1.
The secondary service airbrake system is made up of two subsystems:
a.
Secondary constant pressure system provides continuous air pressure to:
Pedal valve
Rear relay valve
Spring brake air reservoir
Spring parking brake valve
b.
Secondary signal system serves three functions:
Contains air pressure only when operator steps on brake pedal.
Is regulated by various valves to control amount of braking.
Provides pressure to apply rear two service brakes and intermediate and rear axles
stamped with B. Service brakes on rear axle are piggybacked to spring brakes, but operate
independently of them.
2.
The secondary constant pressure system is made up of the following components:
A. LOW AIR PRESSURE SWITCH
- Activates warning buzzer and warning lights when air pressure goes
below 60 psi (414 kPa).
B. PEDAL VALVE
- Allows air pressure from secondary constant pressure system to flow into secondary
signal system when operator depresses brake pedal.
C. SECONDARY AIR PRESSURE GAUGE
- Indicates amount of air pressure in secondary system.
D. 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.
E. 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.
F. INTERMEDIATE REAR BRAKE CHAMBERS
- Converts air pressure to mechanical force which applies
intermediate rear service brake.
G. DOUBLE CHECK VALVE NO. 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.
H. SECONDARY AIR RESERVOIR DRAIN VALVE
- Provides a drain for moisture and air from secondary
air reservoir.
I. 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.
J. STOPLIGHT SWITCH
- As the brake pedal is depressed, switch receives an air pressure signal at
electrical contacts which close to activate circuits to taillights.
K. DOUBLE CHECK VALVE NO. 2
- Allows either primary or secondary signal air pressure to activate
stoplight switch while keeping the two systems separate.
L. REAR-REAR BRAKE CHAMBERS
- Converts air pressure to mechanical force which applies rear-
rear
brakes.
TM 9-2320-272-23-1
0003
0003-40


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