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I have been chasing this problem now for 2 weeks. 1961 Dodge Phoenix with manual drums all around (yes, I know I should just do a disc conversion.) New single reservoir master cylinder, new wheels cylinders front and back, soft lines all look brand new. When you first start out driving the car you have nice pedal feel and car is rolling along great. After driving for 30 minutes or so around town the pedal becomes harder and harder. Eventually you can feel the brakes starting to grab ( rears, not fronts). Today when this happened I hurried up and jacked the back end up when i got home. Wheels were practically locked up. I could barely turn them and wheels were HOT. Cracked the bleeders open and fluid shot out like a firehose. Wheel cylinders released, wheels spun freely. This pressure will release on it's own eventually too because once the I stop driving the car for a few hours the wheels free up on their own. I have replaced the obvious, checked the obvious, bled properly, etc etc. Front wheels no problem, back wheels driving me nuts. What would cause pressure to build in the rear wheel cylinders while driving enough to cause the shoes to grab. And why does the pressure dissipate after it sits for a few hours? What am I missing here. I am no drum brake genius but there are only so many things it could be. I have even considered that the fluid may be getting heated up somehow but I don't see anything obvious with the routing of the brake lines. Any ideas, guys?

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