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Loss of brake air when shut off

when shut off I get air loss on the brake system. I cant hear any leaks, or when sprayed all connections I can find I don't see any leak. Is there a valve that maybe faulty that will not show a leak?

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3 Answers

'''Check the bottom of your foot valve there is a rubber dust cover that can mask the sound of seeping air. Put your hand over the bottom and see if there is any pressure build up. Same thing with the brake valves inside the frame rail.

As far as checking for leaking air lines I advise you to buy one of those sprayers that holds 3-5 gal and make up a solution of dish washing liquid and water and completely saturate all your air lines good. Also spray your brake valves in the dash out side and behind the dash panel too.'italic text''

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How much time are you talking about? What type truck and year?

What happens when you do your air pressure pretrip?

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Any movement in the cab causes air loss, as well as being pushed around by air. But it is a good idea to check all air lines for deterioration, dry rot, rubs or holes. Most likely rubs or holes won't be obviously seen, turn lines, etc to see any deterioration. Should you have an older truck like mine, sometimes you just need to change out old for new and make a time table for doing it again in the future to avoid dry rot, etc.

And don't take your mechanics word for it, check them yourself. I have a great mechanic, but his specialty is the engine, not necessarily all the extras.

My truck, a 1999 Kenworth W900L lost pressure coming into a rest area, (the air went all the way down.) Parked quickly.

It started blowing off more than usual after building up to 130 psi and then blowing down to about 120 psi for about three days prior and was more noisy than usual. It would not build up past 70 psi, after I lost pressure. After the air pressure went down, the air dryer kept blowing out the air, as I tried to build the air back up. I had to call road service.

The result was temporarily by passing the Air Dryer, until I could get to a repair shop to replace it. The two large lines going to the air dryer can be joined with a connector, so that your air pressure will build. You just won't hear anything blowing off. Fortunately, I also have three air tanks I blow off everyday; so I can still clear the tanks.

The Air Dryer is not functioning, if it is constantly blowing out the air. Also when you do replace the Air Dryer, go ahead and get a new one, rebuilding one will cost more money and not worth the effort.

If it had been the Govenor, it would not go over 30 psi, due to it not regulating the air pressure. Evidently, the go out less frequently than the Air Dryers.

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