Jill, The first question I have is to verify when the engine is running the belts are tight. Start the bus, get the air pressures up to normal and shut off the bus. Are the belts tight, and if so, how long does it take for them to become loose. Please don't stick your hands in there it the engine is running.

Once you answer that question we can go from there.

Orren is correct, but one problem is you don't want to mess with the adjustment if the pressure is correct. There is a tire valve up there near the regulator. Chack that with a tire pressure gauge right after shutting off the engine and let us know what that pressure is. I have to look it up, but I think it needs to be 50 to 60 PSI (does anyone know off the top of their head?).