[Richard]

Realizing I have leaks to resolve, I still want to make sure I understand the system. I understand the PPV will shut off to protect air to the critical systems. But, assuming chassis air doesn't drop to 60, then in theory it should stay open and still supply air?

Think of the PPV as a diverter valve, not a check valve. As air enters, all air goes through one of the PPV ports to the brake tanks, after 60 psi is reached, the other port opens to allow air to flow to the supply side. Both exit ports would remain open IF the system pressure is above 60. This is why there are check valves on the brake tanks to prevent the brake tanks from backfeeding a leaking supply side. For some reason, the DOT thought brakes were more critical than potties.

Run this little experiment to watch it work. Turn off the coach, turn off the 120V compressors. Continue to pump the brakes until the brake tanks read close to zero. Operate the door, or bleed the supply side until it is zero. Now crank the engine. You should see the brake tank gauges start to climb, but the supply should remain at zero. The brake gauges should climb until they reach around 60, then you should see them stall and the supply side start to climb. When all three reach 60 they will climb in unison. It's a sanity check to see if the PPV is operating as it should.