Parking brake will not disengage

Yes if it is a Work Horse P32 chassis, it has auto park. Check the oil in the Plastic oil tank in the steel box that is usually mounted underneath the coach on the passenger side behind the transmission. There is a place you can see the level and a black plastic cap to where you fill it. If it is out of oil fill it with automatic transmission oil. I carry a spare rotten green switch as they can cause trouble. If you want more information google "old used bear" he specializes in rebuilding auto park units and has a lot of info about them. Good Luck with your problem!

Check all fuses first.
If P-chassis do not pull the pin .!!!!! There is still pressure on the cable probsbly. You need 2- 19mm open wrenches and slowly lossen tension on the cable and drum brake will release. Of course chock all tires before doing anything. the lock nuts are on the cable at the front of the steel box enclosing the P-Brake mech. There are several posts about the system on IRV2 site. Likely the rotten green switch as is has become known as. All parts are avaiable at a commerical hydralic supply.

If it is the workhorse chassis which is what it sounds like from your post, there is a pressure switch inside the steel box. When you put the shift lever from park to any gear you should hear the pump come on. Do you hear the pump? If so, it is probably the pressure switch. The older ones were green, but I understand the newer versions are brown. If you can confirm you have the workhorse chassis with the auto park system, and the pump comes on you will get more help.

OP said "P-brake button". That would indicate Workhorse P32 chassis with DRUM BRAKE on the end of the transmission.
So the hydraulic release pressure is not over coming the brake apply spring.

To the OP. Chock a tire, then go under your coach and remove the cable pin, near the drum, that applies the E brake. That will allow you to drive to a service center. You will NOT have park or an E brake, so have you co-pilot chock the tires when you stop.

Richard

Crank it, put in into gear and hit the release. Could be the "frozen" the brakes and rotor together with a layer of rust from sitting for a while without being used. Very light attempt to move forward and back will likely break it free if that is the problem.

How long has it been since you've driven it anywhere?

Does you engine start and run OK?

And, as enblethen asked...Ford or Chevrolet.

What chassis, Ford or Chev?