BTW 45-degree double flare in stainless steel is a PITA. I bought this tool from Easton and it's amazing. You don't need it if you're going to use something softer, but it makes stainless easy to do. If you don't want to pick one up, you can send me a USPS label and I'll lend you mine.
https://www.eastwood.com/professional-brake-tubing-flaring-tool.html
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It seems that those AN fittings are acceptable to the FAA and the military for use in 1500psi (operating pressure, not burst pressure) hydraulic & pneumatic systems. Stainless steel AN fittings are used in 3000 psi systems. ??????Neil, I have heard that some people have had street cars fail inspection with AN fittings on hard lines. Specifically on some forums (including this one) and from a reputable shop that replaced the brake system for a car that failed inspection. That said, no one has ever pointed me to any regulation that disallows 37-degree single flares. I read through all of the federal DOT regulations that I could find and there was no mention of flares. I agree that 37-degree single flare (AN fitting) is a lot easier to make and that the there are nicer fittings available. I would have preferred to go that direction, but I didn't want to have a potential issue and tool mentioned above makes it easy to make the double flare. So all of my hard lines are 45-degree double flare and the flex lines are 37-degree single flare.
Fudge!!! I spent at least $100 on extra brake lines already flared, had to bend them (not easy) . Per howards suggestion I changed the in line check valves for wildwood ones, that have different size fittings, so I had to reorder a 60 inch flared stainless steel line and bend it to fit. Broke one along the way. Tha was a royal PIA. Should have bought the stainless steel flaring kit to begin with. Scotts offer would have saved me tons of time and money. But now my lines are all done. No the prettiest bends but done. I am posting a new build thread today.