Stopping the Flow

| September 22, 2009

Finding and correcting braking-system air leaks can keep you safe and in service


The basics
The air-brake system has three sections: the supply portion, the parking brake and the service brakes. The parking brake system has air pressure inside only when the parking brakes are released, while the service brakes have air only when applied.

Regular servicing of the air dryer helps prevent leaks by keeping the various system valves in good condition.

The most basic symptom of system leakage is a compressor that spends too much time running.

You’ll hear the compressor cut in and then hear an audible blast of air as it cuts out.

Another symptom is when you lose all pressure overnight with the engine off.

If you hear compressor cycling while sitting in traffic with a constant brake pressure, or when idling with parking brakes applied, the air-supply part of the system is leaking.

Ideally, the compressor will run only 10 percent of the time. If it’s running 30 percent of the time or more, you have a leak in the supply system.

A small leak won’t show on your air gauges. To check, turn the engine off and depressurize the brake system. Then, tee a gauge into the wet tank. Start the engine and idle it until air pressure is up. If you see a rapid leak down and rise of pressure, the supply portion of the system has a leak. This needs to be fixed right away as it means more system maintenance and repair. To locate leaks audibly, get the rig into an enclosed, quiet area. Run the engine until the air compressor cuts off. Then shut the engine down. Stay off the truck.

1. Examine the compressor discharge line, shown here where it connects to the compressor and at the first connection. Look for cracks in copper lines and for a small residue of moisture and oil. Look at all hoses where a line might have rubbed anything and at all connections. Inspect the compressor unloader and the dryer purge valves, crawling under the truck if necessary. It may help to use a spray bottle of soap and water solution to spray connections. When air leaks into the soapy water, it will bubble.

2. Note that the two valves in the dryer can leak because of dirt. If this occurs, service the dryer by replacing the cartridge, then clean the valves.

3. Check for leaks in the parking brake system. To do this, 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. Check for a brake drum on one wheel that runs hot even when the other drums are cool, which means incomplete release there.

4. If this doesn’t reveal a problem, make sure to 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. Now apply the service brakes. 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.

5. 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. Bendix makes a system called the BBA85 which applies service brake air pressure via a dash switch. 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. Note that the service brakes are the hoses that feed the inner chambers that are closer to the slack adjusters and the connecting plumbing. This includes the relay valves that receive an air signal from the treadle valve and deliver air into the chambers. Check all the connections and the relay valves; 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, make sure the cause is not a leaky brake chamber. Depressurize the service system and disconnect and plug each of the valve’s delivery ports, then restore service brake pressure one port at a time. If plugging one of the ports stops the leak at the relay valve exhaust port, replace the brake chamber that port serves. If the leak continues, replace that relay valve.

Our thanks to Tom Soupal, principal engineer at MeritorWABCO; Chuck Eberling, principal engineer, and David Dennis, senior technical rep, at Bendix; and Randy Petrish, vice president of technical services at 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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