Ken,
The pressure should be 120 PSI, It does not matter whether it gets there at idle or at fast idle. The way you know is when you hear the dryer blow off with a loud pssssst sound. It is more obvious when the engine is at idle rpm. Here is the sequence the air system goes thru if airing up from 0 psi in the air tanks. 1-dash "low air light" and warning buzzer on. 2- park brake will release at 35 to 40 psi, warnings continue. 3- At 65 to 75 psi the warnings will stop. 4- The dedicated rear brakes tank usually fills first, this is the red needle on older coaches or a separate gauge on newer coaches. 5- When the psi gets between 90 and 100, the accessory air tank starts to fill your air bags and raise to "travel height". 6- When both air tanks fill to 120 psi is when the air dryer purges. When driving, you will notice the accessory tank (green needle on older coaches or separate gauge) move from 120 psi down to 90 psi. This is because your suspension is consuming air as necessary to maintain a level ride height. When the governor sees that 90 psi, it tells the compressor to come back on and build pressure back up to 120 psi.
I am not sure what your issue is. It needs professional attention and correction. The last thing want is your rear brakes partially engaged or "dragging" while in transit. Hope this helps, Fred
