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 79 of 481
TM 9-2320-272-10
1-62
Change 2
1-20. COMPRESSED AIR AND BRAKE SYSTEM OPERATION (Contd)
c. 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.
NOTE
When vehicles are equipped with ABS,
they have a new type of rear relay valve
that replaces item I below.
• 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.
NOTE
On vehicles equipped with ABS, the rear
relay valve operates the rear-rear right
hand brake chamber and itermediate rear
right hand brake chamber. The front relay
valve operates the rear-rear left hand
brake chamber and intermediate rear left
hand breake chamber.
• Provides pressure to apply the rear two service brakes and the
intermediate and rear axles stamped with B. Service brakes on
the rear axle are piggybacked to spring brakes but operate
independently of them.
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