Section 12: BRAKE AND AIR SYSTEM
4 X3-45 Commuter PA1593 DOB 2400-2489 Section 12 Updated Sept. 2015
1. AIR SYSTEM
The basic air system consists of an air
compressor, reservoirs, valves, filters and
interconnecting lines and hoses. It provides a
means for braking; operating controls and
accessories, and suspension (refer to Section
16, "Suspension", for complete information on
suspension description and maintenance). An air
system schematic diagram is annexed in the
technical publications box provided with the
vehicle for better understanding of the system.
2. BRAKES
This vehicle uses both the service brake and
emergency/parking brake. The service brake air
system is divided into two independent circuits to
isolate front brakes from rear brakes, thus
providing safe braking in the event that one
circuit fails. Front axle brakes operate from the
secondary air system, while brakes on both the
drive axle and tag axle operate from the primary
air system.
The tag axle service brake operates only when
the axle is in normal ride position (loaded and
down).
Furthermore, the brake application or release,
which is speed up by a pneumatic relay valve (R-
12), will start with the rear axles and will be
followed by the front axle, thus providing uniform
braking on a slippery road. The vehicle is also
equipped with an Anti-Lock Braking System
(ABS), which is detailed later in this section.
The drive axle is provided with spring-loaded
emergency/parking brakes, which are applied
automatically whenever the control valve supply
pressure drops below 40 psi (275 kPa).
3. AIR RESERVOIRS
The air coming from the air dryer is first
forwarded to the wet air tank, then to the primary
(for the primary brake system), secondary (for
the secondary brake system), and accessory (for
the pneumatic accessories) air tanks (Fig. 1).
Two additional air reservoirs are installed on the
vehicle: the kneeling air tank and the parking
brakes overrule air tank.
3.1 MAINTENANCE
Ensure that the wet (main) air tank is purged
during pre-starting inspection. In addition, it is
good practice to purge this reservoir at the end
of every working day. The remaining reservoirs
must be purged at every 12,500 miles (or 20 000
km) or once every year, whichever comes first.
FIGURE 1: I-BEAM FRONT SUSPENSION AIR RESERVOIRS LOCATION 12213
3.1.1 Wet (Main) Air Tank
This reservoir, located above the L.H. wheel of
drive axle in the rear wheelhousing, is provided
with a bottom drain valve. A recommended
purge using the bottom drain valve should be
done every 12,500 miles (20 000 km), or once a
year, whichever comes first.
3.1.2 Primary Air Tank
This reservoir is located above the R.H. wheel of
the drive axle and is provided with a bottom drain
valve (Fig. 1). It is recommended to purge the
primary air tank every 12,500 miles (20 000 km)
or once a year, whichever comes first.
3.1.3 Accessory Air Tank
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