The brake assemblies used are Stopmasters. 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 engaged with and supporting the brake shoes.
The brakes are double-actuated, each of the two actuating systems has one anchor (solid) plunger and one adjustable
plunger. 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.
The Fail-Safe unit is a spring powered brake actuator that assembles piggyback on the air chamber. When 70 psi or
more air pressure is applied against the piston, the spring will be held in a compressed position. When the air pressure is
removed, the spring will push the piston against the diaphragm plate and apply the brake.
The plungers are adjusted automatically. The adjusting bolt is threaded into an adjusting sleeve which in turn is free-
fitted inside the plunger proper. The plunger guide screw is replaced by a hollow cap screw, a spring, and an adjusting
pawl which also serves as the plunger guide. The end of the adjusting pawl has saw-tooth type teeth which engage
corresponding helical teeth on the outside of the adjusting sleeve.
As the brake is actuated; the plunger, sleeve, and bolt move outward and the sloping face of the teeth on the adjusting
sleeve lifts the adjusting pawl (against the spring). When the brake is released, all the parts return to their starting points.
As the lining wears, the plunger stroke and resulting pawl lift gradually increases until the pawl climbs over and drops into
the next tooth space. This time, when the brake is released and the plunger is pushed back in its bore, the upright face
of the pawl teeth causes the adjusting sleeve to rotate and advance the adjusting bolt. This reduces the lining clearance
and the cycle starts over again. The automatic adjuster operates only in forward vehicle direction.