This one had us baffled for a short time. We resurrected my son’s ’65 El Camino that had been in storage for about five years. We installed a new battery, rebuilt the carburetor, and had the car running for a short time before we realized the rear brakes were dragging. The evidence was smoke coming from the rear drums after a short drive on the highway.
We noticed there was residual pressure in the rear hydraulic circuit that kept the wheel cylinder pistons applied. The car was equipped with a rear brake adjustable pressure regulator so we assumed that was the problem. We purchased one from Summit Racing and installed the new valve but that did not solve the problem. Even worse, now very little pressure was being applied to the rear drums. We next looked for a kink in the line was restricting the flow. We could not find one which left the flexible brake hose as the only other possibility. A visual inspection of the hose did not indicate it was bad, but after we removed the hose an air nozzle with 100 psi pressure test could not force air through the hose so clearly it had collapsed internally.
We purchased and installed a new rear brake hose and now the brakes work properly. A close inspection of the old hose revealed small lumps that indicated the hose had separated internally. In the future we plan to replace the other front hoses just to avoid this issue in the future. Another mystery solved. It just took us longer than normal to diagnose the problem.