sometimes the ebrake cables will cause extreme dragging like that. It's been suggested to spray WD40 into the boot where the brake cable enters the drum. check the cable it's self, too. Mine was locking up and the cause was the cable it's self - the boot had broken open where it splits and heads to each wheel - it had gotten dirty and was sticking.
Also, check your springs. if they aren't stout enough, they won't pull the shoes back. I'm guessing that the reason it's locking up over time is because there's already a little drag, and as it get's hotter, it expands and gets worse (just from rubbing on the drum)
I may be wrong, but I think that if your mas cyl is going, it's more likely to loose pressure, not lock up. if your problem is hydrolic in nature, you may also check the junction that the mas cyl goes into. that controls braking pressure - should split it 60/40 front to back. I've heard that when those get mucked up, you'll have some locking too. I think it acts sort of like a check valve, and that's where the sticking happens.
Also, back off on the star wheel a little. you'll need to do that to get the drums off to check it all out - I'd bet on mechanical issues over hydrolic any day...
