Big Rig Basics

John Baxter | October 01, 2011

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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