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.


Air system troubleshooting

Degree of difficulty: 8 out of 10

Tools needed: A spray bottle of soap and water, wheel chocks, and either a helper or a piece of wood cut to hold the treadle valve down to apply the brakes as you inspect the system.


Buy quality relay valves

You may find that you need to replace various air brake system parts if they fail to perform or start leaking air. One especially critical part is the relay valve used at each axle to send air to the brakes when you hit the brake treadle valve.

It always pays to buy high quality parts. One way to do this is to buy only OEM parts from the original maker, such as Bendix, Haldex, MeritorWABCO or another major supplier. If you buy any other brand, make sure the part conforms to OEM standards, and ask for proof. For example, relay valves need to respond similarly to the pressure coming from the treadle valve. If you replace the original with a valve that does not have identical performance, your brake system will be out of balance. The axle served by the relay valve you replaced will come on too early or too late, and the result will be overheated linings, with the potential for premature wear and even a runaway.


Big Rig Basics Tip

James Taylor drives a Western Star and is leased to Oakley Trucking of Little Rock, Ark. He says he’s had little trouble with his air brakes except for the occasional freeze-up in winter at a valve or a 90-degree elbow.

He’s found that the key to heading off such trouble is to service the air dryer. That’s because it gets the moisture out of the system and, if it’s working right, the air will be so dry it won’t freeze up, even in the dead of winter.

Part of the servicing procedure is to replace the seal for the blow-off valve when you hear it cycling erratically. It’s a small O-ring that has to be worked into position. It’s also necessary to replace the desiccant and filter. The desiccant takes the moisture out of the brake system air, and the filter helps strain out oil and dirt. Both are normally contained in a spin-on filter can that you replace just like a fuel or oil filter.

Taylor runs 120,000-130,000 miles a year and services his dryer about once every two years, but other truckers have found different intervals are right for their operation. Highway driving will be easier on the dryer, while any application during which air brake usage is extensive, like city driving, means more frequent dryer service.

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