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 243 of 1313
AIR PRESSURE SUPPLY SYSTEM OPERATION - Continued
The constant air pressure supply is distributed to the primary service airbrake system and secondary airbrake system
through a shutoff and check valve. Air pressure can either be fed from or supplied to another vehicle through the
emergency couplings.
A. FRONT EMERGENCY COUPLING
- When vehicle is being towed, coupling receives compressed air
from towing vehicle's brake system to charge its own brake system.
B. PRIMARY AIR RESERVOIR
- Stores sufficient air pressure to allow operator to make normal brake
applications if system pressure fails or engine stalls.
C. PRIMARY FEED CUTOFF
- Manually-operated valve used to isolate pressure leaks in primary air system
from draining wet tank (TM 9-2320-272-10).
D. PRIMARY AIR RESERVOIR CHECKVALVE
- Prevents backflow of air from primary tank if wet system
develops a leak.
E. SECONDARY FEED CUTOFF
- Manually-operated valve used to isolate pressure leaks in secondary
air system from draining wet tank (TM 9-2320-272-10).
F. SECONDARY AIR RESERVOIR CHECKVALVE
- Prevents backflow of air from secondary tank if wet
system develops a leak.
G. REAR EMERGENCY COUPLING
- When towing another vehicle, coupling allows pressurized air from
wet tank to charge towed vehicle's wet tank.
H. SECONDARY AIR RESERVOIR
- Stores enough air pressure in case constant pressure system fails or
engine stalls. The operator can make normal brake application before running out of air.
I. PRESSURE PROTECTION VALVE
- Performs two functions:
•
Allows air pressure to build to 60 to 65 psi (414 to 444 kPa) before supplying air to auxiliary air-
powered equipment.
•
Closes off auxiliary air system from other systems if an accessory fails and prevents loss of air from
secondary reservoir.
TM 9-2320-272-23-1
0003
0003-38
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