65 Valiant 4th or 5th master cylinder still leaking

I know what you talk about, and same deal for my 65 Newport. Most new MC's have the bottom port w/ a plug that you don't need. I had one MC with only the single horizontal port and it leaked because I over-tightened the "NPT to inverted flare" brass adapter and must have split the cast iron. Easy to do, so perhaps many rebuilt ones have a hairline crack. I fixed it with epoxy and maybe teflon tape, and used silicone. Don't know if glycol fluid would attack the epoxy. I since changed to a modern aluminum 2-port MC on all my 60's cars, which you might consider (read posts). If you were nearby, I would give you a new single-pot MC that came w/ a brake booster (if I didn't sell it, forgot).