Section 12: BRAKE AND AIR SYSTEM
PA1562
16
• Clogged engine air cleaner.
• 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).
• Governor poorly adjusted or defective.
• Restriction in line between governor and
compressor unloading mechanism.
Air pressure drops quickly when engine is
stopped:
• Leaks in compressor discharge valve.
• Leaks in governor.
• 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 axle, 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).
27. AIR BRAKES
27.1 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 14 or 16 square inch effective
area for service. 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.
27.1.1 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. 25). 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,
since all pads are the same. Once removed,
worn pads should be replaced in their original
position.
FIGURE 25: BRAKE PAD CHECK 12117
27.1.2 Caliper Maintenance
Use the following procedure for brake calipers
servicing. The procedure must be followed in
proper sequence to ensure that only needed
repairs or replacements are performed on
calipers. Problems such as hot brakes or
cracked rotors may be effects of sticking
calipers, too-small clearance between rotor and
pad or possible trapped air pressure in the brake
chamber. If any of these symptoms occur,
To Next Page
Loading...