We have a 2006 C2 that the driver must hold the service brake pedal down to release the Parking brake. Now the P brake won't release and it leaks air out of a valve in front of the rear axle???? Any one had this problem?
If I had to guess right off the bat it would be the Air Management Unit. I have replaced quite a few of them, especially in early model C2. If you have access to service link you may be able to confirm this.
Also, does the unit have a wheelchair lift. If so, turn off the switch. Make sure the lift door is shut all the way. We have had some that are slow releasing and they have been a faulty quick release valve back near the rear axle.