I know this is an old thread but I have recently been looking at my and other metal disc brake lines that causes me to open this discussion again. I had 6 of these lines and was getting them cleaned and sorted to go to the zinc plater. The problem (or no problem) with these lines is that after years of use they can become rusty and/or the flare nuts become very burred. Of the 6; I'm left with 3 that have relatively good nuts and no pitting or rust. As Mark pointed out the new replacement lines are black and getting non-black NOS pipes is almost impossible.
Anyway the reason I am resurrecting this thread is because my good friend Rob has a NOS pipe in a bag and I asked to use this as a guide to making sure the bends in my used pipes are the same. Well I'm looking at Rob's very old NOS pipe, and as stated previously (with photo) by Gerard, the same black nuts were on this pipe and there was something else. To my mind this pipe in a sealed bag is not zinc plated. Nope! It's just not zinc looking. I check the original pipes on my bikes and the one on 28 doesn't show the black passivated nuts. This pipe is, as far as I understand, original to the bike and the only intervention by me was to ultrasonically clean it and re-install without doing anything else. It is left in the original rust free finish. My second bike has a rusty pipe but the nuts seem to have residual zinc plate still evident. I also believe this pipe is original to the bike. Both seem to have no black passivated flare nuts.
Of the 3 pipes I intended to get zinc plated, 2 have zinc flare nuts and the other has black passivated. All these brake lines came from Tokyo1 assemblies. Now previously I said I'd never seen a black flare nut on one of these lines and I hadn't until now. I must have just plain and simply missed that detail, as these lines came off existing brake assemblies I have.
My next post is the interesting one.