Hot Air

Will rising underhood heat cause brake air systems to fail prematurely?

"Number three: you have to spec' it with the right air flow and the right size compressor," he goes on. "We're using a BA921 now, from Bendix, and when we have to go with a bigger one we use a BA922. And we make sure we spec' the Bendix air dryer on it. Now we keep it clean without having to rely on the drivers to drain the air tank, like we did years ago. We rarely have to do that now."


McKibben's strategy falls right in line with the advice of the worried experts: check your brake air system thoroughly, and check it often.

"The first checkpoint is opening up the manual drain valves on the reservoirs, whether that's the wet tank or the primary or secondary tanks," says Johnston. "The first checking point is either pulling the lanyard on the drain valve cable or getting under there and opening the drain cock. The mechanic can clearly see if there's moisture coming out.

"Then I would back up and look at the dryers," he continues. "Each dryer, regardless of design, has a purge valve—a discharge, an exhaust, coming out of the dryer, typically on the bottom. A good indication of the health of the dryer and compressor is looking at how much moisture and evidence of oil is coming out of the purge area. If there's more oil than normal, there's something going on with the compressor or possibly the dryer.

"By using my foot valve or reservoir drain cocks, I'd make sure that the compressor loads and unloads on a smooth and consistent basis," he concludes. "That can be done by lowering the pressure in the system and then letting the compressor start to compress air again and watch to see that it unloads properly and doesn't quickly cycle back and load again."


"The compressors are dependent on lubrication oil from the engines, and they also depend on coolant from the engines," says Petresh. "You need to have enough coolant flow, and with these new engines there's a big question mark in our minds as to whether the coolant levels and lubrication levels are going to be adequate. Some configurations of engines provide less flow, or the oil coming in is as hot as the oil going out. That translates into significantly reduced life durability for the compressors.

"Compressors deteriorate and wear out over time, they're going to pass more oil, and that's going to contaminate your air dryers," he continues. "And if the air dryers are totally contaminated, they'll pump that oil downstream."

"The SAE (Society of Automotive Engineers) has a standard of 160 degrees F for air dryer inlet temperature," says Leslie Kern, senior product manager—heavy duty, for SKF. "The higher the temperature, the quicker you're going to have oil carbonizing and coking up the lines before the air dryer, which will make the compressor work that much harder. It will also affect the components within the air dryer; you will get a problem with oil contamination in the desiccant, shortening the service life of the air dryer.

"For us, the key would be to have plenty of pre-cool line between the air compressor and the air dryer, and that can be done through spec'ing or before the vehicle is delivered to the fleet," she says. "The standard right now is at least six feet. If you're going to run hotter you may want to go to eight feet, and you must have at least a steel braided line."


"The OEMs have three options once it is determined that a particular component is at risk," says Sharon Seitz, product manager, Trailer Systems, Bendix Commercial Vehicle Systems LLC. "They can relocate the part. In some cases, the component can be shielded from the heat source. They can upgrade to a component designed to tolerate the higher temperatures. For instance, Bendix released a brake valve that is more tolerance of high temperatures—the Bendix® E-6™ HT valve design which has been released by some OEMs."

Seitz notes that the E-6 HT is specifically designed to handle higher temperatures, and is marked with a metal tag stating 'High Temp Product.' "To prevent premature degradation, it is important to replace that valve with a genuine Bendix HT valve, NOT a standard brake valve," she cautions.

"We're looking at all our designs," Petresh says. "We're looking at different rubber formulations, different sealing compounds, and have been for quite a while. Even 20 to 30 degrees is significant. If you're 20 or 30 degrees below your threshold, and now you're at your threshold, then 20 or 30 degrees is life-or-death."

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