Don't know if this is the answer your looking for but if you have ever flared steel brake lines with out double flaring they split. Why ????? All I know is when I started working on cars back with Fred and Barny I was told double flare so I do and I don't get leaks. I have done repairs on brake systems where someone used copper tubing inplace of steel with compresion fittings. There is probably a writen rule somewhere but I don't have a copy of it either.