I have not had that problem on Stovebolt engines, but, I have not done much/any "plumbing" on my Stovebolt engines in the last 10-15 years.
However, I have been recently installing compressed-air "plumbing" in my garage with new black-pipe and brass-fittings.
They leak-like "Hxxx". I am having to use teflon-tape AND Teflon-goop/paste to seal the fittings. I attribute this to Chinese-attempts to make USA thread sizes? It is very difficult to eliminate leaks above 90 psi.
