Leaky adjustable proportioning valve

"YOU" are not listening. NO pipe thread properly bottoms out like yours is doing. Either the male fitting is metric, or was manufactured wrong, or the female is manufactured wrong.

Pipe threads are TAPERED. This means that as you thread a fitting, the more you run the die down a male fitting, the SMALLER the fitting becomes. Likewise, the DEEPER you run a tap into a female fitting, the LARGER it becomes.

NO pipe fitting, properly made up, should look like yours does. You have 1/8 pipe. In many cases, 1/8 is "precision" enough in fit that they will seal with NO sealer.

That brass fitting should be available at ANY auto parts store. That would be female 3/16 inverted flare X 1/8" male pipe.