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 249 of 1313
PRIMARY SERVICE AIRBRAKE SYSTEM OPERATION
1.
The primary service airbrake system is made up of two subsystems:
a.
Primary constant pressure system provides continuous air pressure to:
•
Pedal valve
•
Rear relay valve
•
Spring brake air reservoir
•
Spring parking brake valve
b.
Primary signal system serves three functions:
•
Contains pressure only when operator steps on brake pedal.
•
Is regulated by various valves to give operator control over amount of braking.
•
Provides pressure to apply front service brakes and the front two service brakes on
intermediate and rear axles stamped with an A. Service brakes on the intermediate axle
are piggybacked to spring brakes, but operate independently of them.
2.
The primary constant pressure system is made up of the following components:
A. PRIMARY AIR PRESSURE GAUGE
- Indicates amount of air pressure in primary system.
B. LIMITING VALVE
- Serves three functions:
•
Regulates signal air pressure going to front brake chambers so rear brakes are applied first.
•
Regulates signal air pressure to front brake chambers so operator has control over amount
of
braking.
•
Releases air pressure in front brake chambers directly to vent in the valve when brake pedal
is
released.
C. FRONT BRAKE CHAMBERS
- Convert air pressure to mechanical force which applies front
service
brakes.
D. PRIMARY RESERVOIR LOW AIR PRESSURE SWITCH
- Activates warning buzzer and warning light
when air pressure goes below 60 psi (414 kPa).
E. PEDAL INTO VALVE
- Allows air pressure from primary constant pressure system to flow into primary
signal system when operator depresses brake pedal.
F. 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.
G. FRONT SERVICE COUPLING
- When vehicle is being towed, coupling is connected to towing vehicle
so the brake systems of the two vehicles work together.
H. ONE-WAY CHECKVALVE
- Allows air pressure to flow into primary reservoir, but prevents it from coming
out if constant pressure system fails or engine stalls.
I. 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.
J. DOUBLE CHECK VALVE NO. 2
- Allows either primary or secondary signal air pressure to activate
stoplight switch while keeping the two systems separate.
K. STOPLIGHT SWITCH
- As brake pedal is depressed, switch receives an air pressure signal which closes
electric contacts turning on stoplight.
L. REAR SERVICE COUPLING
- When towing another vehicle, coupling is connected to towed vehicle so
the brake systems of the two vehicles work together.
M. INTERMEDIATE FRONT BRAKE CHAMBERS
- Converts air pressure to mechanical force which
applies intermediate rear service brake.
N. PRIMARY RESERVOIR DRAIN VALVE
- Provides a drain for moisture and air from primary air reservoir.
O. 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 pedal.
Releases air pressure in rear brake chamber directly to vent system when brake pedal is released.
TM 9-2320-272-23-1
0003
0003-44
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