assuming as you stated that all grounds and wiring is good, if the turn signal and brake light are the same element, and the turn signal works fine but brake is intermittent, as ol' 55 said, turn signal switch may very well be the issue, especially if it is original style (original or new replacement OEM style).
The old style turn signal switches don't always return to center properly and can cause either one or sometimes both brake contacts inside to not make contact and you get a brake light out. I added a lair if indicator lights inside on my 58 that are tied to the rear turn signals/brake light wires so I can tell if one is not on.
