PREVOST H3-41 Maintenance Manual page 445

which will guide you to the most common causes
of problems.
Before performing any test, check all air lines for
kinks or dents, and hoses for signs of wear,
drying out or overheating.
WARNING
When working on or around brake system and
its related components,
precautions should be observed:
Always block vehicle wheels. Stop engine
when working under a vehicle. Keep hands
away from chamber push rods and adjusters
as they may apply when system pressure
drops.
Never connect or disconnect a hose or line
containing air pressure. It may whip as air
escapes. Never remove a component or pipe
plug unless you are sure all system pressure
has been depleted.
Never exceed recommended air pressure and
always wear safety glasses when working with
air pressure. Never look into air jets or direct
them at anyone.
Never attempt to disassemble a component
until you have read and understood the
recommended procedures. Some components
contain powerful springs and injury can result if
not properly disassembled. Use only proper
tools and observe all precautions pertaining to
the use of those tools.
Always clean connecting piping and/or fittings,
and coat pipe threads with Teflon pipe sealant
before installing any
component.
Pressure Build-Up / Low Pressure Warning /
Cutoff Point / Air
Governor Cutout
CONDITION: Vehicle leveled, parking brake
applied.
1. Completely drain
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 85 psi (415 kPa).
the following
air brake system
Filter/Dryer Built-in
wet, primary and
PA1621 Maintenance Manual All Series | Revised June 2020
SECTION 12: BRAKE AND AIR SYSTEM
5. At 85 psi (586 kPa), run engine at full rpm,
then check that build up time to 100 psi (690
kPa) is 13 seconds or less.
6. Air filter/dryer built-in governor cut-out. Cuts
out at the correct pressure of 140 psi (965
kPa).
7. Air filter/dryer built-in governor cut-in. Cuts in
around 122 psi (841 kPa).
For common corrections, refer to the following
check list:
High or Low Warning Cutoff Point
• Perform a telltale light and gage test. Replace
entire cluster if found defective.
High or Low Air Filter/Dryer Built-in Governor
Cutout Point
• Perform a telltale light and gage test. Replace
entire cluster if found defective.
OR
• Repair or replace air filter/dryer as necessary
after checking that compressor unloader
mechanism operates correctly.
More than 13 seconds to build-up pressure
from 85 to 100 psi (585 - 690 kPa) at full
engine RPM
• Perform a telltale light and gage test. Replace
entire cluster if found defective.
• Check compressor strainer or inlet line. If
restricted, clean or replace element or faulty
line.
• Check compressor head or discharge line for
carbonization or restriction. Clean or replace
as necessary.
• If discharge valves leak, pull head and
correct or replace cylinder head.
• If drive is slipping, replace gear.
• If inlet valves are stuck, open or leaking
severely, replace unloader kit, inlet valves
and/or seats as necessary.
• If drain cock is found open, close it.
• Listen for air leaks and repair.
• Redo list to check all items repaired or
replaced.
Air Supply Reservoir Leakage
41