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Air Leak at Foot Valve Exhaust


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As noted in the Midliner project thread I now seem to have an air leak at the exhaust port of my foot valve. I did a hard stop before coming up the driveway and suddenly dropped to 30 PSI on both circuits and couldn't build past that at idle. I was able to rev it up to build enough to release the spring brakes and get up the driveway. I pulled the service side lines off the rear brake cans and got no backflow out of them. With the engine stopped, park brake set, and service brakes not applied, it was still leaking from the foot valve exhaust port. Reconnected the service chambers, chocked the wheels, released the spring brakes, and gave it a few hard, full applications...magically, the leak was way, way less but still present.

I'd read a bunch of other BMT posts before making this one, and seen that a lot of people hose them down with WD-40 or some other very thin lubricant first, before replacing. I figure it's a piece of crud or some internal rusting caused by the previous owner's lack of maintenance on the air system (e.g. not blowing down tanks). It's a pretty new WABCO foot valve (looks like it was put in probably 2015). I'm planning a three-hour trip down to Winston-Salem and back later this week, is this something I should just bite the bullet on and have replaced now, or see if I can give it a workout and free it up?

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With your traveling, a new one sounds good.  

Doing a little driving in the yard may clean it out.  I have taken them apart and inspected the seals and cleaned the bore.  Gave the seals a very slight amount of petroleum jelly and it hasn't caused any other issue in 10 years.  This was on a truck were a direct replacement was not readily available.


It doesn't cost anything to pay attention.

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Shop took a look at it today, mechanic said it was almost certainly crud or moisture from poor maintenance of the air system by the previous owner. They sprayed some air line cleanout/dryout chemical in there and worked it back and forth a few times, more or less no leak after that.

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