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 405 of 481
TM 9-2320-272-10
3-56
3-13. SPRING BRAKE SERVICE
a. General.
Spring brakes on M939/A1/A2 series vehicles lock automatically
and stop the vehicle whenever a large loss of air pressure occurs. Before the vehicle
can be towed away for repairs, the spring in each spring brake must be manually
released. This procedure is performed by operator only in an emergency.
b. Applying Spring Brake.
Switches located on the emergency brake and PTO
levers reduce the air pressure to the spring brake system, resulting in automatic
activation of spring brakes. Testing of these switches can be accomplished by manu-
ally lifting switch and increasing engine speed while in drive.
c. Spring Brake Override.
An air-actuation switch on the dash supplies nor-
mal air pressure to spring brakes preventing them from engaging.
d. Releasing the Spring (Caging Brakes).
WARNING
Make sure vehicle parking brake is engaged and wheels are
chocked before releasing springs in spring brakes. Failure to do so
will result in vehicle rolling out of control, which may cause injury
or death.
NOTE
• Do not lose rubber plugs. After removal, store all four plugs in
map compartment inside cab. Be sure to notify unit maintenance
of where they are stored so they may be reinstalled later.
• If inside of brake chamber is clogged with mud, sand, or dirt, do
not proceed with spring release operation unless the chamber
can be cleared. Notify unit maintenance if chamber cannot be
cleared.
(1)
Remove rubber plug (1) from spring brake chamber (2).
(2)
Visually inspect spring brake chamber (2) for mud, sand, or dirt.
WARNING
Do not remove rim clamp bolt or nut. High pressure inside of
spring brake chamber may result in injury or death if released.
(3)
Remove nut (6), washer (5), and release bolt (4) from storage housing (3).
(4)
Insert T-end of release bolt (4) all the way into spring brake chamber (2)
and turn bolt (4) one-quarter turn clockwise.
NOTE
If release bolt cannot be pulled directly out of spring brake cham-
ber after it has been turned, bolt is properly seated.
(5)
Pull on release bolt (4) to make sure it is firmly holding spring plate with-
in the spring brake chamber (2).
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