[dannymarilyn]

Does the pressure in the brake tank increase went the engine is running?  Have you plugged in to AC power or started the generator and turned on the 120 volt air pump to see if the pressure comes up with the 120 pump?

If it does, your engine mounted air compressor may not be coming cycling on.

Brake pressure is zero until engine is started. Then brake pressure begins to build. At some point (around 70-80 psi) the supply system would begin to
pressurize. Brake pressure is building to 120 or so, but supply pressure never builds.

I just bought the coach 08-25-15 and didn't verify the operation of the electric compressor. I found the switch, but don't have a clue as to what it actually operates
and can't find it mentioned in any of the books.

I'm suspecting there is a solenoid or regulator which 'guards' the air pressure for the brakes and only allows flow to the supply system above a required minimum
for safe brake operation. There has to be something malfunctioning in there or maybe a switched valve?

Where are the isolation valves?

Thanks!
Danny