Mack M359, Manual

Page: 298 / 332

MAINTENANCE, LUBRICATION AND SERVICE
STEERING AND BRAKES MAINTENANCE
panel. When air pressure drops below
517 ± 34 kPa (75 ± 5 psi) in either
system at any time other than vehicle
startup, pull to the side of the road and
determine the problem. If air pressure
continues to drop below 55 ± 5 psi in
BOTH systems, the Low Air Pressure
Warning indicator and buzzer will be
activated if low air pressure occurs in
either circuit.
In tractor applications, the Trailer Supply
Valve (red octagonal knob) will
immediately pop out in the event of a
trailer breakaway or sudden trailer air
line failure, which will apply the trailer
spring brakes. In the event of a slow leak
in the trailer air system, the trailer supply
valve will pop out when system Pressure
reaches 70 psi. This protects the tractor
air system from further pressure loss.
If the Trailer Supply Valve is held in, in an
attempt to override application of the
trailer spring brakes, the Park Brake
Valve (yellow diamond knob) will
automatically pop out and apply the
parking brakes when system pressure
drops to approximately 20–45 psi. The
trailer will pop first and then the tractor.
The air brake system consists of three
main elements:
• The compressor, governor and
reservoirs supply and store the air
pressure.
• The brake application valve controls
the brake application pressures.
• The brake chambers control the
brake mechanism.
Air Brake Operation
CAUTION
Avoid sudden stops. Constant, sudden
stops may negatively affect the
performance of braking and driving parts.
When slowing for a stop, leave the
clutch engaged for as long as possible
to use the braking effect of the engine.
When forward speed has dropped to a
little above idling speed, push clutch
pedal in and brake to a complete stop.
Automatic Slack Adjuster
DANGER
Automatic slack adjusters MUST NOT be
manually adjusted in an effort to correct
excessive push rod stroke, as this
condition indicates that a problem exists
with the automatic adjuster, installation of
the automatic slack adjuster or problems
related to components of the foundation
brakes. These conditions will not be
corrected by manually adjusting the
automatic slack adjusters. Manual
adjustment of automatic slack adjusters is
a dangerous practice that could result in
serious consequences. This practice gives
the vehicle operator a false sense of
security about the effectiveness of the
brakes, and the brakes will likely soon be
out of adjustment again.
Automatic slack adjusters are designed
to automatically maintain proper brake
chamber pushrod travel and
compensate for brake lining wear during
normal use. Manual adjustment of an
automatic slack adjuster should never
be performed except when performing
brake or wheel service (such as backing
off the brake shoes for wheel removal,
brake shoe relining/replacement, brake
drum reconditioning, etc.).
When pushrod travel exceeds
specifications (as given in the "BRAKE
ADJUSTMENT" section of the
Maintenance and Lubrication Manual,
288
X Publication_PC28