I am going to try to reuse the brake line I removed by cutting off the bad fitting and installing a union. I also plan to make a new stainless line just to learn how.
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Chances are that will work. I have definitely done exactly what you are describing.
I am fairly sure that they are steel. I don't believe they are stainless. Couldn't swear to it not being available, but the ones I have seen definitely rust.
Making a new line... The straight pieces of line aren't very expensive. Get yourself two or three pieces and lay the old one on a table and start at one end GENTLY bending or forming the new to match the old.
IF you put even a single kink in the steel line -
it is over. You absolutely cannot use a kinked line, no exceptions.
If a kink happens, use that line to practice bending. That is all it is good for now, but you may as well make it a learning experience.
If you need to make a bend really close to the fitting, I can share some magic with you. Let me know if you need that. AND, if it seems that bending gently by hand isn't working for you - it may be that you need a tubing bender...
Best of luck with your project. It is definitely "doable". It just may take you a little time to work out how to make it work...