Air compressor problem

Simply not true. Boost pressure is only 30-35 PSI. under high load RPM it wont help to build pressure 100 - 110.
Air compressor intake plumbed in the intake manifold only to have clean air and short intake pipe. Allegation that if air compressor intake pipe disconnected from the engine manifold boost will rise wrong as well.
The problem with slow pressure buildup could be just dirty unloader and valves.
Fix is easy. Disconnect air compressor intake hose and discharge side. I will recommend to put rug over discharge port or dirty substances will hit your face. Start engine. Check with your hand or finger pressure out. Spray carb cleaner into intake port until clean. Spray it slowly to avoid flooding and hydro lock. Check pressure with hand after cleaning. This is standard procedure for Cummins ISX.

 

It's about pressure ratio, 30-35 PSI in, means denser air in. Putting air in that is 35 psi higher than atmospheric will reduce the pressure ratio and make the compressor more efficient. Simple math:
Atmospheric to 120 psi - 14.2 psi/120 psi = 0.118
Atmospheric plus 35 psi of boost - 49.2psi/120 psi = 0.41

This does not include adiabatic efficiencies or and temperatures, it's just to simplify pressure ratio. Denser air into any compressor can build the same pressure for a given volume faster. On the extreme, imagine an engine with 120 psi of boost, the compressor wouldn't need to do anything under boost. Cut that in 1/2 to 60 psi and the compressor would only have to work 1/2 as hard to fill the tanks.

I'm not disagreeing with you on why they plumb it to the intake (easy close place to get clean air), for the amount of time you need air under full power, the difference isn't very significant, I'm just saying that in that case of the air leak, boost can increase efficiency of the compressor to get it up to 100-110, where it couldn't keep up with the air leak under no boost conditions.

 

I had a similar problem where I spent over a week trying to figure out the problem, I found one valve sensor under the dash that was leaking, replaced it but was't cured,

I rebuilt the compressor and yes it was a bit better but not completely fixed.
Then I passed by the governor and everything fixed.

The thing I regret is not starting with the simpler stuff first.

According to bendix you start with checking leak, bend lines ,governor ,dryer, then compressor in order .