8 Tips for Repairing Air Brakes

• Posted on Autos.com Editor
• in Auto Repair
• on October 7, 2013

Heavy duty trucks have special braking needs, which require the use of air brakes. Air brakes operate using a belt driven compressor to keep a constant pressure in a set of air accumulators. This pressure is approximately 115 to 120 pounds per square inch. These types of systems are more complex and harder to maintain. There are a variety of problems that can arise with air brake systems. These are the two most common:

• Brake binding/sensitive pedal
• Excessive pedal pressure required

This guide will instruct the reader on how to trace the causes of these problems and repair them.

Care must be taken in all phases of repair, since, unlike hydraulic brake systems, air brakes maintain air pressure even without the brakes applied.

Brake Pedal/Sensitive Pedal

This is a condition that can have a couple of causes. The main case is usually a binding condition in the brake chamber. The brake chamber is a sealed air chamber with a rubber diaphragm with a steel push rod in the middle of it, which actuates the brakes when the pedal is pushed. There is a spring to force the rod back to the relaxed or inactivated position. This spring will occasionally bind and cause the brakes to either stick against the drum or not fully retract. Care must be taken when repairing this, as the system is under pressure and the spring is high tension.

1. Drain the air from the system.
2. Use a spring compressor to hold compression on the spring and remove clips and retainers.
3. Slowly release the tension and remove the spring, push rod and diaphragm.
4. Remove any debris and hone the bore with a cylinder hone. Clean thoroughly.
5. Use brake lube to lubricate the bore and reassemble the spring, push rod and diaphragm into the air chamber.

Excessive Pedal Pressure/Weak Pedal

Air pressure acts in a similar fashion as the power brake booster in a passenger car or light truck. This action makes it so that normal braking action will not require massive amounts of pedal pressure to stop the vehicle. If your vehicle is requiring more pedal pressure than normal, this will usually mean that the system is not building air pressure in the accumulators.

1. There should be an air pressure gauge on the dashboard. Check to ensure that it reads approximately 110 pounds per square inch.
2. If not, make sure the belt on the compressor has not skipped off the pulley. If the belt is on the pulley, then it most likely needs to be tightened.
3. If neither of these actions fixes the problem, then the compressor will need to be replaced if no leaks are heard.

If prudent care is taken during these repairs to follow these instructions, you can safely undertake basic repairs to your heavy duty vehicle’s air brakes.