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Double flare for automotive brake fittings.
Buy a good quality kit that has the double flare buttons with it.
The key to a good kit is the metal quality & longevity of the tube gripping threads in the clamp and the threads and quality of the screw handle. The junk won't hold the tubing well which makes a clean double flare a problem.
If you plan to make more than a couple of flares, get one of these tools. I got mine from Inline (I got the display model discount at Carlisle), but Eastwood and a slew of others all sell the same tool. You cannot make a bad flare with it, and I've done stainless as well as steel lines.