I'm not the expert of the century, but I do know that brakes keep you and your loved ones and neighbors alive.
I have never seen any reference anywhere in the last 4 decades referring to using any seal rather a double-flare for brake lines and fittings.
To the contrary, every source I've ever seen makes it very clear that this is the ONLY way to make and use proper brakes lines and fittings.
I've seen exhaust and muffler shops use compression fittings on brake lines when they "accidentally" cut them removing old exhaust systems. If I found one on any car I owned, I would hunt that guy down and shoot him like a rabid dog!