MOTOR TRUCK SERVICE MANUAL
BRAKES-AIR
STOPMASTER ACTUATING SYSTEM
This picture (as illustrated) shows one of the actuating systems of an air-operated double-actuated Stopmaster Brake
(RDA). All parts are shown in the off position.
This system has an air chamber power unit threaded into
the wedge bore of the plunger housing. The socket in the
end of the diaphragm push rod connects the air chamber to
the wedge rod. The wedge retracting spring acts as the
return spring for both the wedge and the diaphragm. A pair
of rollers are held in place on the wedge head by a retaining
cage. The rollers are also engaged in corresponding slots in
the inner ends of the plungers. The unslotted portions of
the inner ends of the plungers are resting on abutments in
the plunger housing. The outer ends of the plungers are
On a double-actuated brake, each of the two actuating
systems has one anchor (solid) plunger and one adjustable
plunger (as illustrated). On a single actuated brake the one actuating system would have two adjustable plungers. All of
the plungers are retained in the housings and the roller slots are kept in proper alignment by means of guide screws which
engage slots in the side of the plungers.
When the brake is actuated, the air chamber pushes the wedge head deeper in between the rollers. This spreads the
rollers and plungers apart and pushes the brake shoes outward. Initially all the plungers are lifted off of the plunger
abutments and momentarily suspended. As the shoes (linings) contact the drums, the drum drags the shoes and the
suspended plungers around with it. This causes the plunger at the trailing end of each shoe to reseat on its abutment and
thus absorb and transfer the brake torque to the brake support. When the brake is released, the wedge spring returns the
wedge and diaphragm to the off position. At the same time, the shoe return springs push the raised plungers back to their
abutments.
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