How to Change an Air Brake Slack Adjuster

by Grace Mclain

The slack adjuster is an air-driven component that controls the up and down movements of the brake pads. When the air is released to the slack adjusters, the pressure from the air chamber pushes the rod towards the slack adjuster. The S-cam rod inside of the slack adjuster then turns the S-cam that is attached to the wheel hub assembly. Once the S-cam has fully turned, the brake pads will be fully compressed to the brake drums. If the slack adjusters malfunction or quit working altogether, they will need to be replaced.

Step 1

Park the truck in a safe working area and set the air brakes.

Step 2

Put the wheel chocks behind both rear tires so that the truck will not roll

Step 3

Get back in the truck and release the air brakes by pushing in on the air brake handle.

Step 4

Locate the air tank on the side of the truck and slowly open the valve to release any excess air from the system. This will remove all air from the braking system and make the slack adjusters free moving. Leave the handle in the open position.

Step 5

Crawl under the rear of the truck and locate the slack adjuster. Loosen the slack adjuster by turning the adjuster bolt head on the rear of the slack adjuster counter clockwise. This will loosen the slack adjuster up from the S-cam

Step 6

Remove the pin from the connecting rod that connects the slack adjuster to the air brake chamber. Start by pulling the cotter pin out of the end of the rod with the pliers. Then, tap the pin through the end of the connecting rod with the rubber mallet. Pull the pin out and place it with the cotter pin.

Step 7

Remove the locking washer on the side of the slack adjuster with the flat-head screwdriver. This locking washer is what holds the S-cam rod inside of the slack adjuster.

Step 8

Tap the side of the slack adjuster with the rubber mallet towards the end of the S-cam rod until the slack adjuster comes off. Then, pull the slack adjuster off of the S-cam rod and put the new slack adjuster onto the S-cam rod.

Step 9

Reverse the same steps above for installing the new slack adjuster. Once the new slack adjusters are on, go back to the air tank and close the handle.

Step 10

Crank the truck up and allow the air pressure to build back up to at least 120 psi. Then pull the air brake handle back out.

Step 11

Crawl back under the truck and mark the connecting rod where it goes into the back of the air brake chamber with the chalk

Step 12

Go back to the front of the truck and release the air brake handle. Then, go back to the slack adjusters and locate the chalk mark on the connecting rod. The chalk mark should be moved at least two inches away from the back of the air brake chamber.