Need rear brake help!

I have a 2003 tacoma 4x4 xtracab and I've been having problems with my rear brakes slowly starting to lock up on me (after a few miles of driving and using the brakes). I popped the drums and found one of the wheel cylinders leaking, so I replaced it and redid the rear brakes shoes.

I thought this might fix the problem, but it's still there. Drove the truck after doing the fixes and the rear wheels locked up again after a few miles of driving and breaking.

I'm going to pop the drums again and check the star adjuster and parking brake cable/assembly. I checked them before putting the drum on last time and everything seems ok.

Any ideas of what the issue might be? Master cylinder? Brake lines? Proportioning valve? It seems like they are fine when the brakes are cold, but once it heats up the brakes lock on. Maybe somehow the brake fluid isn't getting out the wheel cylinder. Not sure where else to look........

 

http://www.google.com/url?sa=t&rct=...x4CoAg&usg=AFQjCNFVyo8ufxPXhmMtj8NoYm3lb_Vyhg

 

good question week return springs blocked line somewhere possible master cylinder

 

Is your truck lifted? Not that it maybe the problem but if it is you need a BPV bracket.

 

break proportioning valve probems is my guess. could the linkage be bent maybe after wheeling in deep mud or something,but bending it would make it think you have a large load on your truck making rear brakes work less, but it depends on how its bent.Im interested in what fixes this.