Your generation of bus is like mine, the flow is from air dryer to wet tank to dry tank to E-filter to pressure protection valve to the rest of the system. There is one check valve on the output of the air dryer, and one on the input to the dry tank. Pressure gauge reads from the dry tank. Accessory system, including the engine stop skinner valve and the fast idle skinner valve, are after the pressure protection valve, as is the parking brake tank. The inversion valve that controls the parking brake lock action at the DD3 cans is basically fed from the parking brake tank but the pressure that is used to unlock them after the button is pressed comes from the dry tank acting on the main service brake diaphragms so if the pressure is only 85 psi, it's won't be enough to unlock the brakes.
When this happened to me it was a big leak in the accessory system dumping air out. The pressure protection valve is designed to open at around 65 psi, so under 65 psi it is closed to retain air in the dry tank, above 65 psi pressure is passed on to the accessory system so that's when the leak stops pressure from building.
So because it's airing up past 65 I can safely assume the pressure protection valve is not the culprit? (Though I can't rule it out until I get my hands and eyes on it)
Come to find the accessory tank (underneath driver) valve was open so I was hopeful it was the leak, closed it and still the same thing. I'll make some wooden ramps so I can stop digging a small trench to get under the bus (blocked and cribbed of course)
I can kind of hear what sounds like a tank changing air pressure (not a hiss) Hopefully it's just a drain valve