On vehicles that do not get used frequently, moisture attacks everything and it's not unusual for calipers to freeze - rust forms around the piston and does not allow the piston to retract properly.
It sounds like you have done everything - except bleeding the system - your brake fluid got REALLY hot and needs to be replaced.
BTW - also, don't set the park brake if you are going to park the vehicle for more than a few weeks and is possible, take the vehicle 10-15 mile trip each month. I had a couple of classic cars which deteriorated quicker if not used - I got in the habit of taking them for a drive every month to keep all systems functioning.
Gerry
