Air leak- any help?

u took the words out my fingers les2

 

some ecm"s wont open the valve unless the engine is spinning just the key on alone wont do wit some

 

is it losing a lot of air, like to much for compressor to keep up. int has high idle switch in dash to keep from high idle without brakes set, but my money on airdryier.

 

Would it possibly be somewhere in the park brake system, as in:
Lock the brakes, turn ignition off, no leaks;
Crank truck, release brakes, which charges the system, the leak appears.

 

I had problems with my air drier as well.
Sounds like what you are saying.

 

I too think it's something with the fan. However I wouldn't rule out the air dryer. Is your truck is equipped with the Meritor WABCO "System Saver" or Bendix AD-SP air dryer? Those air dryers are "integrated" units and require a protected check valve (usually in the secondary tank) that allows a metered (ususally viewed as a drop to a certain psi) reverse flow of air from the protected secondary reservoir back into the wet tank and air dryer for the purge volume and regeneration. Those valves can and do go bad. And being an internal leak, you wouldn't necessarily hear it. Note that bypassing the dryer wouldn't eliminate the problem because the valve is external of the dryer (on the secondary tank). Unfortunately, I'm most positive the leak would be noticable with the engine off, albeit only for a short time...that is until the valve finally seals off. Word of caution though, with those systems it is normal to see a slight pressure drop in the secondary tank during the purge cycle. I hope that is not what you are interpreting as a problem...with your description so far, I don't think so.