Rear brake seizing..... help!

[six8burban]

What's the common reason my rear drums would lock up? Lock-up as it feels like I'm driving with emergency brake on. It was hard to rotate rear tires while off the ground. I first disconnected e brake cable wich didn't help. Only by opening driver side bleeder valve releaseing a litile fluid out solved the problem. I took the same 3 mile test drive back home and felt it starting to seize up as I got into drive way. I'm thinking the problem is from exhaust heat or wheel rotation? Stainless steel brake lines have about 1 inch clearence from headers as it passes frame rail from proportion vavle. In the past i have let the truck idle for about 30 min before atempting a test drive. I have front disc brakes stock rear drums with internal rebuild kit.......entire brake system is new except emergency brake cable and brake pedal. 68 suburban only have about 6 miles since rebuild.
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