Air Leak

It could also being leaking back in the pump it self, check valves in pump have been know to leak.

You have two hose connections behind the dash that connect to your air gauge and a couple of connections to the parking brake. These are under the kick panel by your left leg while seated in the driver's seat.

I had an air leak that was located at the air gage in the instrument panel where the air hoses go to. This was on a Freightliner Chassis mfg in 2006. While you are outside going crazy the air is leaking from the inside and you will not hear it. I heard mine leaking when I sat in the drivers seat and did some paper work with no radio or anything running.
A second place that I had a leek was in one of the airbags Freightliner found that and replaced it.

Good Luck
Jimr

Thanks for the info. Where was the ball valve located and how did you determine it was the ball valve. My mechanic is going to try isolating different areas tomarrow.

try to section off starting from the engine toward the front of Vehicle. Try this link. This is just a guide.


http://www.suspensionspecialists.com/techinfo/Bepco_Air_Brake_Trbl_Chart.pdf

Moved from Forum Technical Support

That's a big enough leak that you might actually be able to hear it. Good luck - I spent a while today chasing an air leak. It ended up being a ball valve and the soapy water wouldn't find it because of where it was located.