How to Adjust Slack Adjusters on Trailer Brakes

Written by danielle odom
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Proper adjustment of slack adjusters is essential to the functionality of trailer brakes. Slack adjusters compensate for normal wear of break linings. The slack adjuster is an arm attached to a push rod on the brake assembly. Performing a slack adjustment means simply adjusting the push rod in the air chamber to within a predetermined tolerance, thereby maintaining optimal brake performance.

Skill level:

Things you need

  • Wheel blocks or jack stands
  • Piece of chalk
  • Flathead screwdriver
  • Correct size wrench for slack adjuster

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    How to Adjust Slack Adjusters on Trailer Brakes

  1. 1

    Before beginning your adjustment, it is important to block the tires to prevent the trailer from moving. The best way to avoid movement is to raise the vehicle on jack stands during the adjustment. Have the brake system at full pressure, lower the emergency brake, and push in the air valve, thereby releasing the brakes. Listen for any air leaks, as they can decrease your braking ability.

    Using a piece of chalk, mark the push rod where it leaves the air chamber.

  2. 2

    Using a screwdriver for leverage, pull the push rod out of the air chamber, and measure from the chalk line to the air chamber. That shows you the "stroke range"; anything more than three-quarters of an inch would mean the brakes need an adjustment.

  3. 3

    Locate the manual slack adjuster bolt. It connects the scam to the air chamber on the axle housing with the push rod. Using the appropriate sized wrench, turn the slack adjuster. Watch the scam to see the direction it is moving to ensure you are adjusting it correctly. You can tell you are adjusting it the wrong way if the push rod is pulling out of the air chamber.

    Continue tightening until you feel resistance, which means you have drawn the brake linings to the drum.

  4. 4

    Reverse the slack adjuster one-quarter to one-half of a turn to put a little distance between the drum and the linings.

  5. 5

    Pressurise the system and apply the brakes using normal pressure. You should have no more than one-tenth of an inch between the brake linings and the drum, and there should be no more that 2 inches of travel in the push rod stroke.

Tips and warnings

  • Even self-adjusting slack adjusters need to be checked regularly.
  • Grease the adjusters and the cam brackets to increase their service life.
  • When measuring the push rod distance, try to keep each axle at the same measurement to keep each wheel's stopping ability the same.
  • Failure to correctly adjust brakes could result in injury or death to yourself or others.
  • Make sure the vehicle does not move in either direction before working under it.
  • If you can turn the slack adjuster in both directions without any resistance, it may be overly warn; replace warn slack adjusters.

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