Those cute little flare kits are just about impossible to use for anything stainless and even standard materials for tube larger than 5/16". I gave up on those and bought one of these
Professional Brake Tubing Flaring Tool - Eastwood
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Yeah, it's expensive. But it works every single time, does 3/8" stainless no problem. And I've already used mine enough to make it worthwhile. I usually buy the full length prebent fuel lines just because of how long they are and how many bends they have, but I make a lot of my brake lines because bending/routing them is pretty easy.
