Brake problem

[Chushajo]

Hi all, first post on here

I'm in need of some advice. I recently purchased a 2000 forester which I've had my eye on for years. I got a great deal as it's only covered 45000 miles and was really happy. Driving fine for a few weeks. However I went to use it the other morning and the rear drivers-side break is stuck. I don't mean a little bit, I mean fully ceased, I couldn't move the car at all. Ended up having to get a local mechanic to come and pick it up with a truck, figured it was just a stuck handbrake. They really struggled to get the drum off (ended up having to use a hydraulic puller). They replaced the cylinders on both sides, took it for a drive and it was fine, but after it sat for a little while it was completely stuck again on the same wheel. Pedal still has pressure so it can't be the master cylinder, abs light is not on. They believe it could be a failed proportioning valve, which has been ordered. 

My question is, would this be the culprit (it sounds like it could be)? Anything else to check? Anyone had similar issues?