SECTION 12: BRAKE AND AIR SYSTEM
20
PA1621 Maintenance Manual All Series | January 2017
Air pressure doesn't rise to or doesn't
maintain a normal setting:
Defective air gauge (registering incorrectly).
Excessive leaking in air system.
Reservoir drain cock open.
Governor poorly adjusted or defective.
Defective compressor.
Worn compressor or excessive wear on
piston and/or ring.
Air pressure rises to normal setting too
slowly:
Excessive leaking in air system.
Clogged engine air cleaner.
Faulty pressure sensor.
Worn compressor or excessive wear on
piston and/or ring.
Engine speed too low.
Air pressure rises above a normal setting:
Defective air gauge (registering incorrectly).
Faulty sensor.
Restriction in line between governor and
compressor unloading mechanism.
Air filter/dryer built-in governor poorly
adjusted or defective.
Air pressure drops quickly when engine is
stopped:
Leaks in compressor discharge valve.
Leaks in air lines.
Leaks in air system valves.
26. BRAKE OPERATION
The vehicle braking system uses both service
and parking air-operated brakes. The air
system is divided into two independent circuits
to isolate the front axle brakes and the rear axle
brakes (drive and tag), thus providing safe brake
operation in the event that one circuit of the
system fails. The primary circuit is connected to
the drive and tag axle brakes, while the
secondary circuit is connected to the front axle
brakes. The tag axle service brakes operate
only when the axle is in the normal driving
(loaded) position. The spring-type emergency
brakes are mounted on the drive and tag axles,
and will apply automatically if primary system
pressure falls below 60 psi (413 kPa).
The optional parking brake override system can
cancel the parking brakes, enabling the driver to
move the vehicle to a safe parking place. On
vehicles equipped with this option, operate the
system by pushing down and holding the control
knob located on the R.H. side of the driver’s seat
(see "Operator's Manual" for more details).
Furthermore, brake application or release, which
is sped up by a pneumatic relay valve (R-14),
will start with the rear axles and be followed by
the front axle, thus providing uniform braking on
a slippery surface. The vehicle is also equipped
with an Anti-lock Brake System (ABS), detailed
later in this section.
Brake and air system maintenance consists of
periodic inspections. Check all parts for damage
and brake adjustment (refer to subsequent
headings in this section for more details).
Ensure all fasteners are tight (refer to “Torque
Tables” for recommended tightening torques).
27. AIR BRAKES
DISC BRAKES
Knorr-Bremse SN7 disc brakes are used on all
axles. The front and drive axle discs are
actuated by 24 inch
2
effective area air brake
chambers (22 inch
2
for front I-Beam axle), while
on tag axle, the brake chambers have a 14 inch
2
effective area for service brake and a 16 inch
2
effective area for emergency/parking brakes.
The Knorr-Bremse SN7 brakes are supplied with
automatic clearance (slack) adjusters as
standard equipment for easier adjustment.
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