How to Adjust Air Brakes on Trucks


A Class 8 tractor trailer unit can legally haul 80,000 pounds; permitted oversize loads can be much heavier and be contained in a longer combination of vehicles. That's a lot of weight to have traveling down the Interstate at 70 mph, so proper brake functions are crucial.

Things You'll Need

  • Tractor trailer combination vehicle
  • Wheel chocks
  • Chalk
  • Pry bar
  • Open or boxed end wrench
  • Build up the air pressure in the brake system until the dash mounted primary air gauge reads approximately 120 lbs.

  • Chock the wheels. Return to the cab and release the air brakes by pulling both the yellow and red dash knobs out. You should hear a rush of air as the brakes are released.

  • Inspect the trailer brakes for signs of damage or wear. Locate the slack adjuster mounted on the axle near the wheel assembly. Modern trucks may be equipped with either manual or automatic slack adjusters. Adjusting automatic slack adjusters requires that the area to the rear of the truck is clear and you are safe to proceed in reverse for a distance about equal to the length of the attached trailer. To adjust the automatic slack adjusters, drive the truck in reverse and apply the brake pedal, then repeat this operation twice more. In most cases this will successfully adjust the automatic slack adjusters. Another potential method for adjusting automatic slack adjusters is to repeatedly press the brake petal six times to adjust the stroke of the brake push rod. Inspect the slack adjusters afterward to ensure they are properly adjusted to federal Department of Transportation limits for safe operation.

  • Locate the push rod on manual slack adjusters at the rear of the brake chamber. Mark the rod where it enters the chamber and then, using a pry bar. pull the push rod out of the chamber to its furthest point. The brakes need adjusting if the push rod has more than 3/4 inch of travel.

  • Locate the adjustment bolt on the slack adjuster; there will be a sliding ring around the bolt which must be depressed in order to turn it. While observing the S cam on the brake, rotate the bolt so that the S cam moves in the direction of the brake drum. Continue to rotate the bolt until it stops and the brake pads are against the drum.

  • Loosen the bolt 1/4 turn on the trailer brakes. Repeat the process on each of the trailer brakes then move to the tractor brakes and repeat the process again for each brake. When loosening the adjustment bolt on the tractor brake, rotate 1/2 turn. The purpose for the different adjustment setting is so that the trailer brakes engage a moment before those on the tractor. This allows the trailer brakes to engage first and creates better control of the truck during braking.

Tips & Warnings

  • Brakes should be inspected daily during the pretrip and post trip inspection and adjusted when necessary.
  • The steer axle brakes should only be adjusted by a mechanic.

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  • Photo Credit brake cables image by max blain from
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