How to Adjust Air Brakes in a Truck

The air brakes on a truck are designed to compress the brake pads to the brake drums whenever air is applied. Once the air brake button on the dash is pulled out, air travels to the air canister in front of the slack adjuster. The air pressure then causes the slack adjuster to turn the S-cam connected to the braking assembly on the wheels. Once the S-cam turns, it compresses the brake pads to the underside of the brake drums.

Things You'll Need

• Wheel chocks
• Open-end wrench set

Instructions

1. Park the truck in a safe work area and pull the air brake button out to set the rear brakes.

2. Climb out of the truck and place wheel chocks in front of and behind the rear wheels.

3. Return to the truck cab and push the air brake button back in. This will free up the brakes and make it possible for the slack adjuster to move.

4. Crawl under the rear of the truck and locate the slack adjusters behind the rear axle. The slack adjusters are the components that adjust the brake pads on the brake drums. The slack adjusters will have either a 7/16-bolt head or a 9/16-bolt head on the back of each slack adjuster.

5. Place an open-end wrench onto the bolt head on the slack adjuster. Push in on the bolt head with one hand and turn the bolt head clockwise at the same time with the other hand until the brake pads have fully compressed against the brake drums. There is a slit in the dust cover on the rear of the wheel hub assembly that you can look through to see the brake pads and the inside of the brake drum.

6. Turn the bolt head on the slack adjuster counterclockwise 1/2 turn to back the brake pads off of the brake drums. Then remove the wrench from the bolt head. Make sure that the bolt head comes back out of the adjuster. If not, put the wrench back on the bolt head and wiggle it back and forth, and it will come back out and lock into place.

7. Move to the other slack adjusters and follow these same steps for properly adjusting the brake pads to the brake drums.

Tips & Warnings

• The proper air brake adjustment for a truck that is on the ground is to turn the adjuster until the pads have fully compressed to the drums. Then, back off of the slack adjuster bolt head 1/2 turn. If the truck is jacked up, back off of the adjustment bolt head 1/4 turn instead of 1/2.

• Keep your fingers away from the air canister and the slack adjusters when applying air to the brakes. The moving parts can cause serious injury.

• After backing off the bolt head on the adjuster, the bolt head must come back out of the adjuster for it to be able to lock. If the bolt head is left pushed in to the adjuster, the brake pads will automatically back themselves off while the truck is going down the road.

References

• Cars Direct: How to Adjust Air Brakes

Resources

• AutoZone