From the early 90's when I worked in a brake shop and part of my job was making new brake lines. Depending how much salt and other junk, brake lines eventually rust through. Don't remember any that weren't baddly rust pitted around the leak. Usually discovered when you need to maKe a hard stop. Older American vehicles used 3/16'' lines with inverted flare nuts, 3/8''-24. At some point the larger trucks switch to 1/4''. The flares had the outside folded inside to produce a 2 layer flare, double flare. Don't expect to get many flares out of a cheap flare tool.
Not sure when the Europeans started the bubble flare, 10 mm 1.0 thread. In the mean time the Japanese used the double flare, but 10 mm 1.0 thread looking very close to the 3/8''-24 nuts on American cars. By then, I think GM was using mostly bubble flares and I think Ford and Chrysler some too. The shop manual for my 92 Grand Am had a picture of a hand bubble flare tool. Not common.
Using 60'' lines, I would make up reasonable matches for old lines inserting as many flare unions as needed, and finish the end with a double flare. As the bubble flares become more common, we would start with a bubble flare line, cut it in 2, and finish the right lengths with a double flare union, or additional lengths as needed. With a bender and a double flare tool, anybody can do fairly well on making nice work of a brake line. Never risk your life on a compression union in a brake line.
