I like pipe joint compound, aka pipe dope for pipe threads. NPT - National Pipe Thread, or as some old timers said, National Pipe, tapered. That would be the smart guys that realized pipe thread from Brittan and elsewhere was not tapered. Teflon dissolves in the presence of diesel, #2 fuel oil and the like. Teflon is annoying the second time you put it together - the bits left in the female side get pushed into things it shouldn't. Found Teflon bits, tiny, holding up the unloader valve on my air compressor - from the previous owner.
Flares seal on the flare itself, not the threads. Same with compression fittings - seal on the ferrule, not the threads. Never apply dope or tape to flare or compression fittings. I've never seen NPT threads on any brake system. If the threads do not match the NPT standard, you have something else.
