Rear brake lock up after front brake conversion
9/11/12
Customer
In January, or maybe December, I purchased your front disc conversion, featured upgrade with factory style bullet valve PV68 and factory style bleeder master cylinder DBMS 616. Since my 1968 Chevelle was a factory power drum brake car, I re-used the original distribution block already in place on the front frame rail. My problem is that at times the rear drum brakes will sometimes drag as I am driving, heating them up to the point of smoking, and even almost locking locking up. After parking it and allowing the rear brakes to cool, the shoes release and allow normal driving/braking until the next time. It seems that if I brake extremely cautiously the rear shoes will not drag. Can you identify the likely cause of the problem? Te front disc brakes are fine and the rear shoes and front appear to be evenly proportioned when braking except when the rear shoes fail to release from the drums. I was told by Frederick Jensen when I purchased the front kit that there was no other valve required, just the PV 68 valve in conjunction with my factory Distribution Valve on the front frame. Please respond with your thoughts that might help with the problem.
Support
Hi Charles, The overwhelming majority of the time a master cylinder piston that is preloaded causes this issue. The rod that comes out of the brake booster and pushes in the piston of the master cylinder being too long causes this. When the piston of the master cylinder is preloaded, it usually is does not cause enough brake drag to be noticeable at first, but causes enough heat over time to make the brakes drag. Usually the fronts will drag as well with this problem, but your rear brake shoe adjustment may be causing the rears to heat up first.
You will need to either space the master cylinder of the brake booster or shorten the rod that comes out of the brake booster. Please see this article for instructions. http://www.getdiscbrakes.com/knowledge-base/Brake_Drag_Prevention