Master cylinder leaking?

Hello
On my 66 slant dart, sporting the original style brake system (drums , 9in, cast iron single master cyl), I am having to replace fluid every few weeks, about 3/8in drop. At the front (towards front of car) on the master cylinder there is a threaded plug that goes up from the bottom. It is maybe 1/2in threaded plug. I am not sure what would ever get connected there, but is has a slow drip coming from it. I have tightened it to the point where i feel I might ruin the threads, but it doesnt seem to make a difference. The leak is slow, gathers, then drops straight onto the dust cover on the steering joint, thus the dust cover is completely deteriorated.

Any ideas how to stop this leak? I don't suppose there is teflon tape that can withstand brake fluid? Or ???