and two brake chambers are assembled to the front and rear
axle. The brake chamber movement through the slack adjuster
causes the camshaft to rotate and force the brake shoes against
(1) Attached to the slack adjusters are two
camshafts-one right-hand and one left-hand.
(2) The "S"-shaped part of the camshaft is
positioned between the rollers on the brake shoes.
Actuation. When the brake is actuated, air is
applied to the brake chambers and the diaphragm rod
moves the stack adjusters rotating the brake camshaft
against the two rollers on the brake shoes. This action
forces the brake shoes against the brake drum musing
friction to slow or stop the vehicle. When the brake is
released, spring action returns the brake shoes to release
Brake Air System (Fig. 4-58). Compressed air is
transmitted from the towing vehicle through the service
and emergency air lines. When the engine of the towing
Brake Interlock (M970 except M970 TC-0843, TC-
vehicle is running, air pressure of 90-120 psi is kept in the
0844, and TC-103thruTC-178). The brake interlock system
system through the emergency airline. Compressed air is
is activated when the door on the hose reel cabinet is open.
routed into the rear reservoir through the emergency
When actuated, the interlock valve sends air form the rear
relay valve. From the rear resevoir, the air enters the
reservoir into the service air line. This gives a service signal
to the emergency brake relay valve. The relay valve releases
air from the rear reservoir to the brake air chambers to apply
(1) The front reservoir is for the fail-safe brakes.
A pressure protection valve between the two reservoirs
system connects into the air brake system.
maintains approximately 75 psi of air in the front reservoir
4-32. General, M967A1, M969A1, and M970A1
for release of the fail-safe brakes.
Including M970 SN TC-0843, TC-0844, and TC-
103 thru C-178.
(2) When there is enough reserve air in the front
reservoir, the spring brakes can be released by opening a
Description. The semitrailer brakes are air-operated
cam-type brakes. Two brake camshafts, two slack adjusters,
Figure 4-55. Fail-Safe Unit (M967,
M969, and M970
SN TC-0843, TC-0844, and TC-103 thru TC-178).