Any emergency doors or hatches open? Have you been trying to eliminate wiring? The newer busses are equipped with interlock systems that set off alarms and/or will not start or release brakes if any doors or hatches are open. Likewise, if you try to circumvent these systems it can cause the same result.
If you have a wheelchair lift model, the outside door must be fully latched to disengage the parking brake. There is a micro switch in the lock receiver. No alarm will sound (like the roof exits do) just no brake release.
This system cuts off the pressurized air supply to the yellow plunger with two solenoid valves under the hood near to top of the firewall (or under the floor in my case).
I have a similar issue! Help! I have disconnected the emergency doors and windows, I thought I had ground all the wires, Gracie started, but now the parking brake is locked on! Any ideas of how to bypass that?!