TM 5-3805-254-20-1
ORGANIZATIONAL TROUBLESHOOTING - CONTINUED
MALFUNCTION
TEST OR INSPECTION
CORRECTIVE ACTION
69.
PARKING BRAKE DOES NOT HOLD.
WARNING
Safety goggles must be worn when working with air lines to prevent personal injury.
Ensure that vehicle is on level ground and that rear wheels are chocked to prevent vehicle from
moving or severe injury to personnel may result.
When performing parking brake troubleshooting, follow procedure exactly. Release of trapped air
inside brake chamber can cause brakes to apply suddenly, causing injury to personnel.
NOTE
Check AIR PRESSURE gage for air pressure between 90 and 130 psi. If specified air pressure is
not present, start engine and check that air pressure in air system is between 90 and 130 psi (TM
5-3805-254-10). Shut down engine (TM 5-3805-254-10).
Step 1.
Chock rear wheels of truck. Check for operation of push rod (1) and slack adjuster (2) as follows:
have assistant pull PARKING BRAKE control knob (3). Check to see that push rod (1) and slack
adjuster (2) are moving.
a. If push rod (1) and slack adjuster (2) are not moving, go to step 2.
b. If push rod (1) and slack adjuster (2) are moving, adjust rear brakes (page 2-973).
CAUTION
Care must be taken when working behind instrument panel to prevent damaging wires or components.
Step 2.
Check parking brake control valve (4) for output air pressure as follows: using 5/8-inch open-end
wrench, loosen output hose fitting (5) six turns, push in PARKING BRAKE control knob (3) and
listen for hissing noise.
a. If hissing noise is not heard, replace parking brake control valve (4), notify Direct
Support Maintenance.
b. If hissing noise is heard, pull out PARKING BRAKE control knob (3). Tighten output
hose fitting (5), using 5/8-inch open-end wrench, and go to step 3.
Step 3.
Check front relay valve (6) for exhaust of air pressure from bottom as follows: have assistant push
in PARKING BRAKE control knob (3). Check for exhaust of air pressure from exhaust vent (7) on
bottom of front relay valve (6).
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