SPRING BRAKE SERVICE
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.
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 manually lifting switch and increasing engine speed while in drive.
Spring Brake Override. An air-actuation switch on the dash supplies normal air pressure to spring brakes
preventing them from engaging.
Releasing the Spring (Caging Brakes).
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.
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
Remove rubber plug (1) from spring brake chamber (2).
Visually inspect spring brake chamber (2) for mud, sand, or dirt.
Do not remove rim clamp bolt or nut. High pressure inside of spring brake chamber may result in injury or
death if released.
Remove nut (6), washer (5), and release bolt (4) from storage housing (3).
Insert T-end of release bolt (4) all the way into spring brake chamber (2) and turn bolt (4) one-quarter turn
If release bolt cannot be pulled directly out of spring brake chamber after it has been turned, bolt is
Pull on release bolt (4) to make sure it is firmly holding spring plate within the spring brake chamber (2).