Funny, I just ran into this same problem before I saw this this thread, only mine is a 68. Went right to the concluison the master cyl piston was being held out by the rod as I just put the original booster back on. Thought of the same thing, loosen the master from the booster and check. Well problem was still there-did not even have to touch the brake and car would come to a pretty qiuck stop. Tapped the brake pedal a few times and all of the sudden problem gone?!?! Long story short found MC piston is randomly hanging about a 1/4" from full return to retaining clip, only when letting it return slowly on bench. If I bottom it and just let go it fully returns. Cylinder was JUST SS sleeved by Karps and I installed new NAPA rebuild kit-and put new RPV in the rear port. What gives? There's no ridges/bumps/scores in the sleeve-they hone them after install anyway. Kit comes with new secondary piston & spring but primary is still original. Could it be the primary spring is shot? Thinking of just getting a re-built unit and gutting it for the parts??? Any ideas??? Also if I get the rebuilt unit, are the guts of the Bendix & Delco units the same?