65 Valiant 4th or 5th master cylinder still leaking

That switch looks like an old style brake light switch, activated by pressure rather than one under the dash actived by the movement of the pedal lever. These may have 2 different ports types: one underneath for a brake line fitting and the other with a pipe thread or similar for the switch.

What are you plugging your 2nd port with? Have you looked into both ports with the line and the plug out? Are they both the type of port where brake lines will plug into, where there is an inverted flare at the bottom of the port? Or is one a tapered pipe thread or some other not-a-brake-line type of port?

I have had MC's where both ports were brake fitting types, and I had to take a brake line fitting and a short stub of brake line, weld the line shut, and use that to plug that extra port.

You can't plug a brake port line with a non-brake-line-fitting type of plug. It sounds like your plug is not right.