Have you replaced the rear brake hoses? I had a similar problem, Pulling my hair out to find it. Rebuilt the calipers..still sticking. Went so far as to replace my left rear caliper with a new unit..was still sticking. I finally found out that the rear brake hose metal sheathing under the rubber had rusted to the point it was causing a restriction. The pedal works fine with the couple of hundred psi applying the brakes but wont retract because of the restriction not letting fluid flow back.