1-11. Brake System (cont'd)
The brake air chamber push rod extends to contact the air chamber yoke. This contact turns the slack adjuster which
is connected to the yoke and is splined into the splines of the camshaft.
The camshaft turns as the slack adjuster turns, thus forcing the lining of the brake shoe against the brake drum.
When the brake pedal is released, a drop in pressure causes the relay valves (5 and 6) to release the compressed air
from the semitrailer braking system. With the air released, the brake return springs pull the brake shoes away from
When the air pressure in the semitrailer brake system falls below 60 psi (413.7 kPa), the ratio relay valve (1)
automatically releases emergency air from the spring brake sections of the air chambers (7).
The spring brake section of the air chamber contains an emergency brake application spring. As the emer-
gency air exhausts this section of the air chamber, the spring is released.
The camshaft turns as the adjuster turns, forcing the lining of the brake shoe against the brake drum.
The middle air reservoir (XM1063 only) (4) acts as a surge tank to accept exhaust air from the
non-pressure side of the service chamber in order to prevent water from entering the system during
In event of an emergency stop resulting from a loss of air or a complete failure of the braking system, a brake
caging procedure may be used to move the semitrailer off the traveled portion of the highway.
The brake caging procedure is to be used for this purpose only (refer to para. 2-27).