Section 12: BRAKE AND AIR SYSTEM
PA1553
13
23. AIR SYSTEM TROUBLESHOOTING
The following list has been designed to help in
troubleshooting some of the most common
problems in the air system and main causes.
For air brakes troubleshooting, refer to “Air
Brakes Troubleshooting” in this section. For
more troubleshooting information, refer to the
manufacturer's brochures annexed to this
section.
Air pressure doesn't rise to, or doesn't maintain,
a normal setting:
o Defective air gauge (registering incorrectly).
o Excessive leaking in air system.
o Reservoir drain cock open.
o Governor poorly adjusted or defective.
o Defective compressor.
o Worn compressor or excessive wear on
piston and/or ring.
o Air pressure rises to normal setting too
slowly.
Excessive leaking in air system:
o Clogged engine air cleaner.
o Worn compressor or excessive wear on
piston and/or ring.
o Engine speed too low.
Air pressure rises above a normal setting:
o Defective air gauge (registering incorrectly).
o Governor poorly adjusted or defective.
o Restriction in line between governor and
compressor unloading mechanism.
Air pressure drops quickly when engine is
stopped:
o Leaks in compressor discharge valve.
o Leaks in governor.
o Leaks in air lines.
o Leaks in air system valves.
24. 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 40 psi (276 kPa).
Furthermore, brake application or release, which
is speed up by a pneumatic relay valve (R-12),
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 “Specifications" for
recommended tightening torques).
25. AIR BRAKES
25.1 DISC BRAKES
Knorr-Bremse SN7000 disc brakes are used on
all axles. The front and drive axle discs are
actuated by 24 square inch effective area air
brake chambers, while on tag axle, the brake
chambers have a 16 square inch effective area
for service brake and a 16 square inch effective
area for emergency/parking brakes. The Knorr-
Bremse SN7000 brakes are supplied with
automatic clearance (slack) adjusters as
standard equipment for easier adjustment. For
more information on disc brake components and
maintenance, refer to the manufacturer's
brochure at the end of this section.
25.1.1 Disc Brake Pads
Brake pads have to be checked on a regular
basis depending on the vehicle operation. The
remaining thickness of the pads should never be
less than 3/32 in (2 mm). To check pad
condition without removing the wheel, verify the
position of guide bushing (6) relatively to guide
sleeve (4) (see Fig. 21). When guide sleeve is in
alignment with guide bushing, brake pad
thickness has to be checked more precisely with
the wheel removed. When replacing brake pads,
all four pads on an axle have to be changed at
the same time. There is no inner or outer pad,
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