Help Find Master Cylinder Flared Plugs

Kuz said
If you can't find a 9/16-20 inverted flare plug, you could make it work with a 3/8-24 plug and this adapter. The 3/8-24 plug should be easy to find.

This is what I am considering. Wasn't sure if it would leak or not. It shouldn't be this hard. GM guys have it so easy.