39.1 Release the parking brakes on the vehicle
by pushing in the parking brake control valve
knob. See Fig. 9.7. Check the air governor
cut-in and cut-out pressures. Run the engine
at fast idle. The air governor should cut out
the air compressor at approximately 125 psi
(862 kPa). With the engine idling, make a
series of foot valve applications. The air gov-
ernor should cut in the air compressor at
approximately 95 psi (655 kPa). If the air
governor does not cut in and out as de-
scribed above, it must be adjusted to these
speciļ¬cations. If the air governor cannot be
adjusted or repaired, replace it before oper-
ating the vehicle.
39.2 Check the air pressure buildup time as fol-
lows:
Release the parking brakes on the vehicle
by pushing in the parking brake control valve
knob. With the air system fully charged to
125 psi (862 kPa), make one full brake ap-
plication and note the air pressure reading
on the gauge. Continue to reduce the air
pressure by moderate brake applications to
a maximum of 90 psi (620 kPa), then run
the engine at governed rpm. If the time re-
quired to raise the air pressure to 125 psi
(862 kPa) (from the pressure noted after one
brake application) is more than 30 seconds,
eliminate any leaks, or replace the air com-
pressor before operating the vehicle.
39.3 Check the air pressure reserve. With the air
system fully charged to 125 psi (862 kPa),
stop the engine and note the air pressure.
Then make one full brake application and
observe the pressure drop. If it drops more
than 25 psi (172 kPa), all areas of leakage
must be eliminated before operating the ve-
hicle.
39.4 Check the air leakage in the system.
With the parking brakes applied, the trans-
mission out of gear, and the tires chocked,
charge the air system until cutoff pressure of
125 psi (862 kPa) is reached.
f421624
Fig. 9.7, Parking Brake Valve Knob
Pretrip Inspection and Daily Maintenance
9.21
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