Mike,
I would assume that the air system on my 06 PT would be about the same as your 05, with that said: I have one big air tank in the rear and one of the same size in the front, I believe that from the Air/Dryer it feeds both tanks and then from the tanks to the HWH leveling/ride manifolds and then to the air bags of the suspension system, I have three manifolds in the rear and one in the front.
The braking system comes off the air tanks in a separate line to the control valves through the protection valve to the brake cans. That's the basics of how the system is set up. Two completely separate systems one is suspension and the other braking.
I have never found a leak in the braking system only in the manifold valving and air fittings for the leveling system. Does yours leak air all the time, brakes set/released, does it loose air when applying the pedal? There is probably a 100 or more fittings and valves that could leak air.
The only way I have found leaks is to raise the coach up and block the frame for safety and get in and soap any and all fittings, valves, solenoid valves. I have found many push to connect air fittings leaking, I have replaced these with Parker Brass Air Brake fittings.
There is no easy way that I have found after all these years with trucks to find air leaks, trial and error, soapy water, and listen for leaks.