The air brakes on a tractor trailer are essential in keeping the truck from rolling. The air brakes are controlled by the air pressure from the truck. When the air brakes are applied, the slack adjuster turns the S-cam. The S-cam then rotates the drum brakes until the brakes are fully compressed to the inside of the drums. Adjust the air brakes on a constant basis to keep the brakes from wearing and ensure alignment.
Park the tractor trailer in a safe work area with a level surface.
Apply the air brakes by pulling out on the air brake handle. Walk to the rear of the tractor and place the wheel chocks behind each rear tire of the tractor.
Go back to the front of the truck and make sure the air pressure is at least 120 PSI. If the air pressure is not at 120 PSI, wait for it to build up to 120, then push the air brake handle back in to release the air brakes. Shut the engine off.
Crawl under the rear of the tractor trailer and locate the slack adjusters that are connected to the rear axle housings. The slack adjusters are the components that rotate the brakes up and down.
Look inside the opening of the dust cover on the back of the wheel hub assembly. Locate the drum brakes. The brakes should be about 1/2-inch away from the bottom side of the drum.
Locate the bolt head on the back side of the slack adjuster. This bolt head can be compressed inward to allow the slack adjuster to turn. Position the 1/2-inch drive ratchet with a socket onto the bolt head.
Push the bolt head inward until it stops. Then, turn the bolt head clockwise until the brakes are fully compressed to the bottom side of the drum. Shine the flashlight through the opening on the dust cover to make sure that the brakes are fully compressed to the bottom side of the drums.
Turn the slack adjuster counterclockwise, quarter of a turn, to set the brakes the appropriate distance from the bottom side of the drums. Then, wiggle the bolt head back and forth until the bolt head comes back out and locks in place.
Crank the tractor engine and wait for the air pressure to build back up to 120 PSI. Then, pull out on the air brake handle to set the brakes.
Turn the engine off and remove the wheel chocks. The air brakes on the tractor trailer are now properly adjusted.
The proper adjustment distance for the tractor trailer is a quarter of a turn. The proper adjustment distance for the tractor is a half of a turn.
Do not adjust the brakes more than a quarter of a turn away from the drums or the brakes might not be able to reach the bottom side of the drums to stop the trailer. This can cause a severe accident.