Air leak- any help?

You'd hear the check valve going off.

Do you by chance have one of those automatic grease systems on your truck? Or possible a filtration system for the oil?

 

There should be a small line that comes off your govenor and runs down to your air drier. Spray it with your soapy water from one end to the other it may be leaking and if it is it will only leak when the gov send's the signal to the drier to purge when the gov reaches high high limit until it bleeds down to low limit where the comp kick back in. It only has to leak the air out of that small line down to 90-110 psi what ever your low limit is and then it will start the cycle all over again. Thats why it is probaly purging so often. And when you shut the engine off it leaks down to where the gov hits low limit and then it should quit, thats why it dosen't leak when the engine is shut off. Hope this is the problem because it will be a easy fix for you.

 

Which Bendix AD-? They made plenty--AD-1, AD-2, AD-4, AD-9, AD-SP, AD-IP, & AD-IS. The latter four are the most prevalent these days. Does the dryer have a spin-on cartridge? If so, it would be one of the latter three mentioned. The AD-IS, a self-contained (non-integrated) unit, is a spin-on style and has a purge reservoir that looks like an old vacuum brake booster attached to it. The AD-SP is the only one that I know of from Bendix that requires the protected check valve to be installed in the secondary tank.

 

on that int mine had two lines comeing off the gov. one to the airdryier one went into the dash. we tried bypassing the dryier but it kept building air until reg valve on compressor poped off. im sure theres away to do it but i just put on another dryier

 

By-pass the air dryer and your problem will magically disappear.

 

Sleepless nights post reminded me of a problem another KW owner from our terminal had; the stainless braided line from the compressor, (main line), connected to a woven, wire braid line behind the engine. This line looked good from the outside, but when he hit it with the soapy water, it turned into a bubble factory. The inner rubber hose started falling apart, losing a lot of air, and plugging up the air dryer with small rubber bits.

 

Definte possibility there. That has happened to my hubby's truck (company truck that is) twice.

 

No you wouldn't. The check valve is simply an internal "one-way" valve in the air system plumbing--not a "pop off" safety valve. The special check valve I'm speaking of is a "one-way" with limited "contraflow". The leak would be strictly internal of the system and you wouldn't hear a thing.

Good question though. Something else to consider.

 

Sorry AG thought you was talking about the pop off valve. If it was the one way valve you'd think the whole system would leak down not just the secondary side?

 

Two things come to mind. My other truck fan hub developed an air leak but only when the engine was running so when you tried turning the key on with out starting the truck to check for a leak there was none, as soon as the engine started the leak started too.
The other thing is I have seen is the spring brake chamber leak internally and cause the problem you describe. If you disconnect the application side hose from the chamber and then release the brakes, if the chamber is bad, air will leak out of the fitting where you unhooked the hose causing the pressure loss. The truck brakes will still act normal except for the air loss while going down the road.