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Rear brakes sticking

I have a 1968 el camino with a 327 and a powerglide. I have noticed that the rear brakes have been sticking. I replaced the wheel cylinders and bled the brakes, but lo and behold they are sticking again. (I have narrowed it down to the rears) I noticed that the cylinders will expand, but not retract properly. Is this an issue with a worn out master cylinder? Or?

P.S. this is an all stock car, drums all around.

Jack up the rear wheels, start the engine, shift to drive and rev to 40 MPH. Apply the brakes repeatedly until they ‘stick.’ quickly open the bleeders to see if pressure is being held in the system. If so, the rubber hose from the frame to the chunk is likely collapsed internally. If not remove the drums to check that the primary and secondary shoes are not reversed.

Take apart the rear brakes, and look at the backing plate. On the backing plate you’ll see where bosses are stamped where the brake shoes ride. Sometimes a burr can develope on that part of the backing plate where the brake shoes gets hooked and won’t release.

When doing drum brakes, it’s always a good idea to sand or file these bosses smooth to remove any burrs and then apply some caliper brake grease to prevent the brake shoes from hanging up.

Tester

I should have given a little more info, the new wheel cylinders that I just put in don’t retract after they are actuated (when I have the drum off, and everything else connected) It seems to me that there is some sort of pressure release that is not happening, beings that the shoes do not retract after the pedal is released. it’s like there is something not allowing the presure to release quickly, eventually they retract, but it takes about 30 seconds.

IIRC, this ELC would have front disc brakes and rear drum brakes. If so I would check the proportioning valve which limits the pressure to the rear brakes. Check to make sure it is not stuck and not allowing brake fluid back to the reservoir.

Of course, look at the brake hose connecting the body line to the rear axle T fitting as Red Knox posted.