How to Adjust Type M Air Brakes

Type M air brakes are also known as manual slack adjusting air brakes. The air brakes are adjusted by a manual slack adjuster that keeps the brakes adjusted the proper distance from the brake drums. When the air brakes are applied, the manual slack adjuster turns the S-cams, which in turn applies the brake shoes to the brake drums. Once the drum brakes begin to wear down, you will need to adjust the manual slack adjuster to keep the brake shoes the proper distance from the brake drums.

Things You'll Need

• Wheel chocks
• Standard open-end wrench set
• Flashlight

Instructions

1. Pull your vehicle into a flat, safe working area. Apply the air brakes and leave the engine running.

2. Place the wheel chocks behind and in front of each rear wheel.

3. Release the air brakes and turn the engine off.

4. Slide under the rear of the vehicle and locate the slack adjusters that are mounted to the rear axle. There is one manual slack adjuster on the driver side of the rear axle and one manual slack adjuster on the passenger side of the rear axle.

5. Locate the adjustment bolt on the back side of the manual slack adjuster. The adjustment bolt will be either a 9/16-inch bolt head or a 7/16-inch bolt head. Position a ratchet and a socket on the adjustment bolt. Push the head of the ratchet onto the adjustment bolt to release the spring-loaded locking mechanism that surrounds the adjustment bolt. This will allow you to turn the adjustment bolt.

6. Turn the adjustment bolt clockwise while pushing inward on the head of the ratchet until the adjustment bolt stops turning. Once the adjustment bolt stops turning, it means that the brake shoes are against the brake drums.

7. Shine the flashlight through the back side of the wheel hub to ensure that the brake shoes are fully against the brake drums.

8. Turn the adjustment bolt counterclockwise 1/4 to 1/2 turn to set the brake shoes the proper distance from the brake drums. Gently wiggle the head of the ratchet back and forth until the adjustment bolt and the locking mechanism retracts out of the slack adjuster and locks in place. Remove the ratchet and socket from the slack adjuster.

9. Move to the other slack adjuster and repeat the process.

Tips & Warnings

• If the slack adjuster makes a clicking noise when adjusting the brakes, this is normal.

• Make sure that the air is released from the brakes or the slack adjuster adjustment bolt will not turn.

• Be sure to use the wheel chocks to prevent the vehicle from rolling while working under it.

References

• CarsDirect: How to Adjust Air Brakes
• ICBC Road Sense: A Professional Guide to Adjusting Air Brakes.

Resources

• New Brunswick Public Safety: Air Brake Manual
• Auto Zone: Free Online Repair Guides