The 45-degree double flare is no where mandated by any Federal document. However, if you are building brake components in the U.S. that will be sold in interstate commerce, the long are of the law in the form of the DOT Federal Motor Vehicle Safety Standards come into play. For example, if you look at FMVSS 106, which is the DOT standard for automotive brake hose, you will find references to various SAE specs, which if you follow the train long enough, will get you to components like rigid steel brake tubing and fittings that incorporate the 45 degree flair. The DOT standards, like their SAE counterparts have labeling and performance requirements for components like brake hoses. You will need to have that DOT and/or SAE label as proof of conformance to the standards. As the 45-degree double flare has a long history of satisfactory usage in brake applications it would be wasteful for a potential supplier to chose some other system, such as AN 37 degree single flare. It will also be costly, as the potential supplier will have to qualify his system with testing at certified labs in order to get the DOT seal of approval. Why go to the extra trouble when you can use the tried and true 45-degree double flare readily available for any brake line component? Now if you are just building your own kit car, you can use any tube flare system that strikes you fancy, unless your state DOT has other thoughts.
