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Automatic Slack Adjusters
The trailers automatic slack adjusters provide the means
for routine brake adjustment to compensate for lining
wear. Inspect slack adjusters every 2,000 miles to assure
correct operation.
Operational Check (See Figure 4-11)
1. Block wheels to prevent vehicle from rolling.
2. Check that the push rod is fully retracted, apply air to
release spring brake.
3. Turn adjustment hex counterclockwise to create an
excessive clearance condition. (A ratcheting sound
will occur.)
4. Make a full service brake application. On release,
allow sufficient time for brake to fully retract. During
the brake release, observe rotation of the adjustment
hex (attach a wrench on the hex to make this
movement easier to see). This rotation indicates that
an excessive clearance condition has been
determined by the slack adjuster, and it is making an
adjustment to compensate. On each subsequent
brake release the amount of adjustment and push
rod travel will be reduced until the desired clearance
is achieved.
5. The push rod stroke should be 1-1/2" to 2" with an
100 to 105 PSI service brake application.
6. Measure the movement of the push rod from the
completely released position to the applied position
by marking the push rod where it exits the air
chamber before and after application.
7. If the brakes have been running tight, the control arm
location should be checked.
If the adjuster appears not to be operating, check
the other brake components for proper function
and eliminate any binding. Recheck the automatic
slack adjuster. If the adjuster is not functioning,
the unit must be replaced because failure of
proper adjustment function will result in loss of
brakes.
Figure 4-11: Slack Adjuster
MAINTENANCE AND LUBRICATION
WARNING
4-17
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