Hot Air

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

Twenty degrees doesn't sound like much at first, but it can really make a difference. If we're talking about outside temperatures, it's the difference between a coat and a short-sleeved shirt. Move that same 20 degree shift under the hood of an already hot heavy-duty truck, and you're talking some serious heat.

It's those 20 added degrees in underhood heat, brought about by the new 2007-standard diesel engines, that have some fleet maintenance managers worrying about their brake air system components.

We talked to some brake experts to find out if worries about premature wear and failure on 2007-spec' trucks are legitimate—and, if they are, to find out what fleet maintenance managers can do to address the issue.


"I'm not sure how widespread the fear is," says Randy Petresh, VP of technical services for Haldex. "I'm concerned about it, and I've had a number of conversations with a variety of people both at fleets and at the OEMs themselves, and I know there are quite a few of us who are concerned. We know the underhoods are higher, the operating temperatures of the new engines are higher, and we know there's a lot of stuff under the hood that is affected by higher temperatures."

To a certain degree, Petresh is fine with that—his company has already done a great deal of product development and testing in preparation for the 2007 heat wave—but there are limits to what is physically possible.

"We all have thresholds of pain, so to speak, when it comes to upper temperatures," he explains. "If you're below those thresholds, you're happy. Once you start creeping up closer and closer to your threshold, then the potential exists that you're going to breach it at some time or another. And we're running close."


"There are some things that fall in the category of capability, like seals, for instance; those are one of our biggest concerns," Petresh says. "There is no such thing as a rubber that will handle all circumstances. You can have one rubber that works well in high temperatures, one that works well in cold, one that will work well in oil, one that will work in water, but there isn't anything that will work in all of them. So everything is a very complicated compromise when it comes to seals. And of course there are millions of seals in there, and a lot of them are in the brake system."

"Our biggest concern is the foot valve, because it's got multiple seals," says Paul Johnston, senior director of ArvinMeritor's compression and braking business unit. "Many of those are what we call dynamic seals, which are very sensitive to compression and loss of sealing capability. The tractor protection valve is a little more static, and we've got room to play with some of the effects of that high temperature. As the temperatures get hotter the rubber gets harder and starts to shrink. You start losing that contact with the metal bores.

"Five or seven years on the valves will eventually get to a point that they have what we call compression set, where the rubber gets less elastic and you get more heat wear," Johnston says. "Eventually you get to the same level of leakage, but that's five to seven years out instead of the first year, let's say, under these higher operating temperatures."


Ken McKibben isn't as concerned as Johnstone or Petresh... yet. "Oh, I'm not concerned about it now," he says. "But I could be, because it's going to jump up 20 degrees."

The senior vice president for field maintenance for Penske Truck Leasing, McKibben hasn't seen any reason for concern so far. With 213,000 trucks spread out over 700 locations, he just makes sure his technicians stick to the basics.

"I'd break it down into three parts," he says. "Number one, know the system. The technicians need to know and understand the system and how it works.

"Number two, you need to understand how to keep it dry and keep it well-serviced," he continues. "Now, there aren't a lot of service points on these, as long as you've got the right specifications.

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