MAINTENANCE AND LUBRICATION
4-19
Automatic Slack Adjuster
The semitrailer 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.
CAUTION
Operational Check
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.
WARNING
Replacing Slack Adjuster
1. Chock wheels to prevent vehicle from rolling.
Release spring and service brake. Air chamber push
rod must be
fully released.
2. To maintain a fully released parking brake, a
minimum of 105 psi reservoir pressure must be
maintained. If air pressure is not available the spring
brake must be manually caged.
3. Remove the existing slack adjuster and clevis - DO
NOT REMOVE EXISTING JAM NUT
4. Install the new clevis (with 1/2" pin) onto the pushrod
up to the jam nut -DO NOT TIGHTEN JAM NUT.
5. Fit the installation guide over the cam splines so the
1/2" pin slots face the air chamber.
6. Swing the guide into the clevis until the appropriate
slot totally engages 1/2" pin.
7. Observe the guide pointer arrow:
If the guide pointer is above the clevis pointer, adjust
clevis CCW for alignment.
If the guide pointer is below the clevis pointer, adjust
clevis CW for alignment.
8. Reposition clevis until the guide pointer aligns with
the clevis pointer.
9. Verify by engaging 1/4" pin through the clevis and
guide.
10. Tighten jam nut to 50 ft.-lbs. torque min.
11. Remove the guide from cam shaft.
12. If the push rod threads extend through the clevis
more than 1/16", remove clevis and cut rod to length.
13. If the push rod is not fully engaged in clevis body,
install a new push rod - cut to length.
14. Install the slack adjuster on the cam shaft.
15. Rotate the manual adjuster shaft CW until the slack
adjuster arm holes align with the clevis. Install 1/2”
and 1/4” pins and cotter pins.
The installation guide must be used when
installing or reinstalling automatic slack adjuster.
Failure to do so may result in improperly adjusted
brakes which may cause brake damage or lead to
brake failure.
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.
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