Air LEAK

good chance your leak in an air bar or a air line. I had same situation a while back, heard the air rushing thru the brake dash valve, but it was just the sound of air, not the source of the leak.

 

Do you have any air bags underneath the cab? My 99 pete was losing it's air through those. Maybe spray the all the airbags with water...

 

Check the foot valve under driver floor board

 

A spray bottle with a some soap and water in it can help locate it.
Spray everything that has anything to do with air and watch for bubbles.

 

i got the same problem havent found it, nor have i put a guud effort. but do know i have air windows. ive been betn my self its in those air window lines!

 

If you have trouble finding your leaks you can put food coloring into your air tanks and the leaks will show the color you use. Red or yellow work good.

 

You need a very quiet place. A truckstop won't do. No traffic, no idling engines, no a/c units, no airplanes flying overhead. Take a length of coolant hose or old garden hose and put one end to your ear and the other end you will pinpoint the the leak with.
I say you need a quiet place so you can get a general direction of where the leak is just from your ears. Then use the hose. I can pinpoint which side of a fitting is cracked just from the hose and not even eyeballing it.
I have better luck with the hose than soapy water.