Big Rig Basics

John Baxter | October 01, 2011

Stop, look and listen

Try these simple tests for finding and fixing air leaks

If you hear the compressor cycling while sitting in traffic with a constant brake pressure or when idling with parking brakes applied, or if you lose all pressure overnight with the engine off, the air supply part of the system is leaking. The compressor should run only 10 percent of the time. If it’s running 30 percent or longer, the air supply system is leaking. This needs to be fixed right away or more system maintenance and repair will be necessary.

1. Get the rig into an enclosed, quiet area. Run the engine until the air compressor cuts off. Then, shut down the engine. Stay off the truck.

2. Examine the compressor discharge line. Look for cracks in copper lines and for a small residue of moisture and oil. Look at hoses where a line might have rubbed anything, and at all connections, even under the truck. Look at the compressor unloader. Spray soap and water solution on connections. Air leaks will bubble.

3. The two valves in the dryer can leak because of dirt. If this occurs, service the dryer by replacing the cartridge, and then clean the valves.

4. Park the truck on a level spot and securely chock the wheels. Have a helper engage and disengage the parking brake repeatedly so you can watch the spring brakes on each axle retract. All the spring brakes should retract rapidly and at about the same speed and distance. One retracting slowly or partially means a leak nearby in the parking brake air circuit.

5. If this doesn’t reveal a problem, walk around and crawl under the vehicle to use soapy water to check all the parking brake hoses and connections with the parking brakes released. The parking brake air system consists of the four outer chambers on a tandem tractor or trailer axle and connecting plumbing.

Have an assistant repeatedly apply and release the service brakes. A slow application of one wheel’s brakes, or one axle, may indicate a significant air leak in the line or chamber serving that wheel or in the plumbing related to that axle.

If the leak is too small to detect by applying and releasing the brakes, cut a board to the right length to fit between the front of the seat and the treadle valve to hold the brakes on with about 85 psi apply pressure. Make sure the parking brakes are still released.

6. Again, walk around the vehicle and crawl under it to trace all lines, using soapy water. The service brakes are the hoses that feed the inner chambers closer to the slack adjusters and the connecting plumbing. This includes the relay valves that usually mount on the frame. Check all the connections and valves, and look at hoses and check for leaks where a hose may have rubbed against something.

7. If air is leaking out of a relay valve exhaust port, the cause can be elsewhere and hard to find. Have a technician explore and correct the problem.

Our thanks to MeritorWABCO, Bendix and Haldex for help in developing this article. We also thank G.L. Sayre Peterbilt/International for allowing us to take photos.

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