Is this the post you're referring too?
This is what it says about flares.
Know your flares
There are four (more or less) flaring styles in common use for brake systems. British cars have a bubble flare (aka Girling flare) backed up with a male swivel nut or a 45 degree double flare backed up with a female swivel nut. Metric cars have ISO bubble flares, where the pipes and threads are metric sizes rather than inch. Detroit iron has a 45 degree double flare backed up by a male threaded nut. Most (non-British) race cars are plumbed with AN (aka JIC) type single flares - a 37 degree single flare with a backup sleeve and inch threaded swivel nut (some people make a double flare here, which is useless overkill and may lead to failure; see below). Lastly, some brake fittings use tapered pipe fittings.
Take the easy one first: tapered pipe fittings are not really a positive seal under adverse conditions. They may do the job for a street car, but they certainly have no place on a race car.
The bubble flare is used with a male swivel nut, and seals at the bottom of a drilled and tapped hole, with a nice angled bottom. While it can usually be resealed, it has a limited lifetime - there's no good way to get back the deformation that was crushed out for the first seal, short of remaking the flare from scratch.
Making such a flare is easy: if you are in possession of a standard 45 degree double flaring tool, the bubble flare is what results after the first half of the operation. Simply stop there, and you have the bubble flare which will seat nicely at the bottom of the hole. If you continue, inverting the form tool and finishing the job, you then have the more familiar double flare used by Girling and the US automotive industry.
The SAE 45 degree double flare usually has a male-threaded tube nut that bears directly on the OD of the flared tube- so you need a double flare to help control galling that can result in stress cracking right at the flare. In short, you need "give" there. Problem is, the deformation that results is kind of irreversible, so the next cycle or two will result in your having to use astronomical torques to keep the flare from weeping. Worse is trying to use a single flare in an SAE flare nut and seat, and worse still if the seat is brass- the flared tube is sqaushed from both sides, even as it is deformed by the nut galling on it. The brass seat deforms and work hardens. It may seal once, with a ton of torque and some luck. It's not recommended practice - it's not even a good idea.
Racers (and aircraft, which is where the system originated as the "Army-Navy" or AN standard in WWII) use the single 37 degree flare. The AN single flare is still a concave flare, but its 37deg angle seals by stretching, not squashing. The tube is supported by a separate sleeve that the female-threaded tube nut bears upon. This isolates the flare from the torques imparted by the nut. So rather than trying to get a seal despite the presence of rotating torques and the resulting galling, you press the flare between precisely-machined (steel!) seat and precisely-machined support sleeve. The sealing area under compression is at least double that of the SAE flare. An additional bonus is that the OD of the nut is a lot larger than the 3/8" of an SAE nut, which means you won't kill as many trying to get the proper sealing torque. (Even so, you should always use a proper flare nut wrench on any tube nut.)
The SAE stuff was designed to go together once on the assembly line, and then be "immortal", as defined by Detroit. It's pretty good at it, too! The AN stuff is designed for field serviceability, long fatigue life, and a level of bulletproofness the SAE never considered. A further, Very Strong, recommendation is that single flared AN fittings are the only thing that Carroll Smith will suffer to put on his race cars.
The reason for harping on repeatability and multiple mate-demate cycles is that, to the best of my knowledge, I have never once put something on the car, and had it stay put on. I always forgot something, or broke something during the season, or needed to swap out something because it was at the end of its service life. While the double flare has that nice "squish" feeling as you tighten the flare nut the first time, the AN fitting has the same torque requirement for the second mating cycle as the first. That's where the reliablity across multiple cycles comes from.
Never use compression fittings for brake plumbing.