Master cylinder leakage

That port is for a brake pressure warning switch. It gets plugged if car is not equipped with a warning light.
I've been really frustrated with the quality of pipe threads in many parts of late. Threads not cut cleanly, not deep enough (which translates to width also with a tapered pipe thread), etc. I would try cleaning the port really well, and using a different brass plug (not iron) in the port, with a dab of copper sealant.