Installing a new proportioning valve on my 66SS with several threaded lines. Do you use any sealant, tape or anything to prevent leaks..?? Or just tighten them in??

Bob in CT

Bob, All brake lines have a flare end with flare nut and they do not need anything but tightening. If they leak when tight, then there is a problem with the seat or the line flare. Steve

Brake lines have a double flare.

And if you happen to be using replacement stainless lines, you'll have to really tighten them.
The double flare on the stainless tubing doesn't conform to the flare seat as easily as mild steel.

Flaired tubing connections exist because of not needing to use any sort of sealant.

when doing the final tightening on the new fittings, just keep tightening and loosening them until you can't get it any tighter. every time you tighten it, it will turn just a little farther.