You need to get someone who is familiar with air systems to look at it. Highest failure rate items are:
Governor (under $25)
Air dryer: cost depends on what dryer you have-- a FACTORY reman (i.e. the company that built it, not cheap knock-off)
Yes, you could have a massive leak. But if you start the coach and go to high idle for 2 minutes and then shut it off, you would hear the leak just walking around the coach.
And, as already posted, zero air pressure means the parking brake IS on! It is spring applied and air released. No air-- it IS on.
