k. Air Chamber (M127). Air chambers (fig. 4-39) are mounted on brackets adjacent to the internal wheel
brakes in front of the forward-rear axle and at the back of the rear rear axle. These air chambers convert air
pressure into mechanical motion to operate the slack adjusters when applying brakes.
l. Air Chamber (All Models except M127). These air chambers (fig. 4-38) are installed and secured with
the two hydrauIic master cyIinders to mounting brackets secured to chassis frame crossmembers. Compressed
air admitted into the air chamber acts against a diaphragm to which a push rod is attached. Movement of
this push rod in turn causes a corresponding movement of a piston in the hydraulic master cylinder thereby
converting air pressure into hydrauIic pressure for actuation of the brakes.
m. Emergency Relay Valve. The emergency relay valve on M 127 semitrailers is mounted on the semi-
trailer air reservoir. The emergency relay valve (fig.4-40) on all models except M127 semitrailers, is mounted
on a crossmember of the chassis frame at a point above and between the axles. The valve performs the same
basic function on all of the above semitrailers. The valve directly controls the service brakes on the semitrailer.
It speeds brake action by releasing air from the air reservoir, on the semitrailer directly to the air chambers.
This eliminates the loss of time that would result if sufficient air to operate the brakes had to travel from an
air reservoir on the towing vehicle to the air chambers. Also this valve controls the flow of air to and from the
semitrailer air reservoir, and automatically applies the brakes if the semitrailer breaks away from the towing
vehicle or if there is a serious leak in the emergency air line.
o. Air Reservoir. The air reservoir (figs. 4-34 and 4-41) on M127 semitrailers, is mounted on brackets
attached to the chassis frame between the suspension mounting brackets. The air reservoir (figs. 4-35 and
4-41) on all models except M127 semitrailers, is secured by mounting clamps mounted on the left longitudinal
frame rail just forward of the front-rear axle. The air reservoir is connected by a tube to the emergency relay
valve and provides a supply of air on the semitrailer through the valve for applying the brakes. It is equipped
with a drain cock for draining accumulations of moisture and for releasing air pressure in the semitrailer
brake system, in the event of locked brakes.
a. Brake Adjustments.
(1) General. Brake adjustment to compensate for
normal wear is termed "minor adjustment".
Following a rebuild or when new linings are
instalIed,each brake shoe must be adjusted
to center brake shoe arc in relation to the
brakedrum. This is called "major adjustment".
A satisfactory brake adjustment cannot be
obtained unless wheel bearings are in proper
brakes when drums are hot.
(2) Model M127.
Release brakes. Jack up axle so that wheels may be rotated freely.
2. With wrench, depress spring lock on adjusting worm shaft of brake adjuster and turn worm
shaft until wheel cannot be turned freely by hand. Back off on shaft just enough so wheel
can be turned freely by hand. After completing adjustment, make certain that head of
shaft is in position to fit into slot of lock and that lock is in position to prevent further
turning on shaft.
Repeat this procedure with brake at opposite end of same axle. Make adjustment of each
brake as uniform as possible. Repeat procedure on brakes on other axle.