Section 12: BRAKE AND AIR SYSTEM
PA1553
22
Pressure Build-Up / Low Pressure Warning /
Cutoff Point / Air Filter/Dryer Built-in
Governor Cutout
CONDITION: Vehicle leveled, parking brake
applied.
1. Completely drain wet, primary and
secondary air reservoirs only.
2. Start engine and run at fast idle. Low
pressure warning lights should be "On".
3. Start checking pressure at 50 psi (344 kPa).
4. Low pressure warning lights and buzzer
should go off at or above 60 psi (415 kPa).
5. At 85 psi (586 kPa), run engine at full rpm,
then check that build up time to 100 psi (690
kPa) is 30 seconds or less.
6. Air filter/dryer built-in governor cut-out. Cuts
out at the correct pressure of 123 psi ±3
(847±21 kPa).
7. Air filter/dryer built-in governor cut-in. Cuts
in around 110 psi (758 kPa).
For common corrections, refer to the following
check list:
High or Low Warning Cutoff Point
o Perform a telltale light and gauge test.
Replace entire cluster if found defective.
High or Low Air Filter/Dryer Built-in Governor
Cutout Point
o Perform a telltale light and gauge test.
Replace entire cluster if found defective.
OR
o Repair or replace air filter/dryer as
necessary after checking that compressor
unloader mechanism operates correctly.
More than 30 seconds to build-up pressure
from 85 to 100 psi (585 - 690 kPa) at full
engine RPM
o Perform a telltale light and gauge test.
Replace entire cluster if found defective.
o Check compressor strainer or inlet line. If
restricted, clean or replace element or faulty
line.
o Check compressor head or discharge line
for carbonization or restriction. Clean or
replace as necessary.
o If discharge valves leak, pull head and
correct or replace cylinder head.
o If drive is slipping, replace gear.
o If inlet valves are stuck, open or leaking
severely, replace unloader kit, inlet valves
and/or seats as necessary.
o If drain cock is found open, close it.
o Listen for air leaks and repair.
o Redo list to check all items repaired or
replaced.
Air Supply Reservoir Leakage
CONDITION: Full pressure, engine stopped,
parking brake applied
1. Allow at least 1 minute for pressure to
stabilize.
2. Stop engine, then check air pressure gauge
for 2 minutes. Note any pressure drop.
3. Pressure should not drop by more than 3 psi
(20 kPa) per minute.
For common corrections, refer to the following
check list:
Excessive air loss:
o With the primary air system at normal
operating pressure (95 - 125 psi (655 - 860
kPa)), coat all air line connections and
pneumatic components with a water and
soap solution. Bubbles will indicate an air
leak, and none should be permissible.
Repair or replace defective parts.
o Listen for leaks and correct as required.
o Redo test to check all items repaired or
replaced.
Brake System Air Leakage
CONDITION: Full pressure, engine stopped,
parking brake released.
1. Apply service (foot) brakes, allow at least 1
minute for pressure to stabilize.
2. Hold down foot valve for 2 minutes while
observing air pressure gauge on the dash-
board.
3. Pressure drop should not be more than 4 psi
(27 kPa) per minute.
For common corrections, refer to the following
check list.
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