(2) When there is enough reserve air in the front
(5) Each actuator has one solid plunger and one
adjustable plunger. The adjustable plunger is used to
reservoir, the spring brakes can be released by opening a
adjust brake shoe-to-drum clearance.
Actuation. When the brake is actuated, the push
(3) A fill valve is installed on the roadside of the
rod of the brake chamber forces the wedge head between
semitrailer to permit filling of the front reservoir
the rollers. The rollers and plungers move outward and
push the brake shoes against the drum causing friction to
slow the vehicle.
Service Brake. When the towing vehicle is
attached, the foot brake operates the brakes on both the
(1) As the brake shoes contact the drum, the
semitrailer and the towing vehicle.
drum drags the shoes and the extended plungers around
with it. This action causes the anchor plunger (at trailing
(1) Applying the brake pedal delivers air through
the service air line to the emergency relay valve.
is transmitted through the anchor plunger to the brake
(2) In response to the service signal, the
emergency relay valve releases air from the rear reservoir
(2) When the brake is released, the wedge spring
to the brake chambers. Air pressure causes the wedge in
returns the wedge and diaphragm to the released position.
each brake actuating assembly to move forward and push
At the same time, the shoe return springs push the raised
the brake shoes against the drum.
plungers back into their plunger housings.
(3) Releasing the brake pedal stops the service
Automatic Adjusters. The adjustable plunger of
signal to the emergency relay valve. The emergency relay
each brake actuator has an automatic adjustment feature.
valve closes the supply from the reservoir and releases air
Helical teeth on the outside of the plunger sleeve engage
from the brake chambers.
with saw-tooth-type teeth on a spring-engaged adjusting
Emergency Relay Valve (Fig. 4-54).
increases the speed at which the brakes apply and release.
(1) As the brake is actuated, the plunger sleeve
The relay valve is attached to the rear reservoir above the
moves outward with the plunger. The teeth on the plunger
sleeve lift the adjusting pawl against the spring.
(2) When the brake is released, all the parts
(1) The emergency relay valve releases air from
return to their starting positions.
the rear (primary) reservoir to all brake chambers in
response to the service signal from the towing vehicle.
(3) As the brake lining wears. the plunger stroke
and the resulting pawl lift gradually increases. Eventually,
(2) The emergency relay valve will also cause the
the pawl will climb over and fall into the next tooth space.
fail-safe brakes to apply automatically if the semitrailer
This time, when the brake is released, the upright face of
breaks away from the towing vehicle.
the pawl teeth causes the adjusting sleeve to rotate and
advance the adjusting bolt.
spring chambers which are assembled piggyback on the
(4) The automatic adjuster operates only when
brake air chambers. One each is installed on the air
vehicle is moving forward.
chambers at the four comers of the tandem axle assembly.
The fail-safe unit provides parking and emergency
transmitted from the towing vehicle through the service
and emergency air lines. When the engine of the towing
(1) Air pressure of approximately 65 psi is needed
vehicle is running, air pressure of 90-120 psi is kept in the
to release the fail-safe units.
system through the emergency air line. Compressed air is
routed into the rear reservoir through the emergency
(2) Loss of air pressure will release the heavy
relay valve. From the rear reservoir, the air enters the
spring and push the piston against the diaphragm plate of
the air chamber. This force is applied through the wedge
assembly and plungers to the brake shoes.
(1) The front reservoir is for the fail-safe brakes.
A pressure protection valve between the two reservoirs
(3) The fail-safe units can be manually released if
maintains approximately 75 psi of air in the front reservoir
for release of the fail-safe brakes.