Well, one cause might be that the caliper grease just finally gave way, a rebuild of the caliper might be needed also? just a guess. I personally would not figure the proportioning valve is bad seeing as almost all ( if not all ) of it's functioning parts are internal free from harsh conditions.

 

I open it up today and I couldn't get the pads off without destroying them because they were rusted on. I checked the piston and it worked fine against a block of wood.

The only thing I can think is that someone had this car and let it sit for a long time and it just rusted the pads. Once they drove it again the piston wasn't strong enough to break the rust off.

I got new rotors and new pads in the rear and the car is smooth now.