A safety, valve is located on top of the first
brakes applied, cover the edges of the diaphragm and
reservoir and consists of an adjustable spring-loaded
bolt with soapy water to detect leakage. If leaks are
bail-check valve in a body. It is used to protect the
present, tighten the bolts uniformly until the leaks stop.
system against excessive pressures. normally set at
Bolts should not be tightened so that the diaphragm
approximately 150 psi.
shows signs of bulging or distortion.
any moisture buildup in the system. Moisture in the
Replace the diaphragm if it is worn or leaking.
system prevents the brakes from actuating smoothly.
Replace the boot if it is worn or cracked. With the
The brake chamber (fig. 7-45) converts the energy
of the compressed air into mechanical force to operate
the brakes. When the brake pedal is actuated, air under
pressure enters the brake chamber behind the
diaphragm and forces the pushrod out against the
return spring force. Because the yoke on the end of the
pushrod is connected to the slack adjuster, this
movement rotates the slack adjuster. brake camshaft,
and cam to apply the brakes.
When the pedal is released, air is forced from the
brake chamber by the brake shoe return spring acting
on the linkage. After the shoes reach the fully released
position, the return springs acting on the diaphragm
causes it to return to its original position in the
When performing maintenance of the brake
system, check the brake chamber alignment to avoid
binding action. Check the pushrod travel periodically,
and when necessary. adjust the brakes so that pushrod
travel is as short as possible without the brakes
dragging. The pushrod length should be adjusted so
that the angle between the center line of the slack
adjuster and the brake chamber pushrod is 90 degrees
when the pushrod is extended to the center of its
The slack adjusters (fig. 7-46) function as
adjustable levers and provide a means of adjusting the
brakes to compensate for wear of linings. Air pressure,
admitted to the brake chamber when the brake pedal is
depressed, moves the slack adjuster toward the
position indicated by the dotted lines.
The entire slack adjuster rotates as a lever with the
brake camshaft, as the brakes are applied or released.
Turning the adjusting screw makes the brake
adjustments necessary to maintain proper slack
adjuster arm travel (shoe and drum clearance). This
action rotates the worm gear, camshaft, and cam.
expanding the brake shoes so that the slack caused by
brake lining wear is eliminated and the slack adjuster
arm travel is returned to the correct setting. The
movement of the cam forces the brake shoes against
the brake drum. Friction of the brake lining against the
drum stops the turning movement of the wheel. When
the brakes are released, the brake shoe return spring
pulls the shoes back to a DISENGAGED position.
There are numerous brake valves used in an air
brake system. These valves either apply or release air
Figure 7-45.Brake chamber.
Figure 7-46.Slack adjuster.