Stainless is difficult to work with. Different tubing materials can require different flare angles, be certain the double flare tool you choose is correct for the material used. If you could by a MBZ line of a longer length one end would already be flared and you'd have the nuts to go with it. Shorten the line and make it match the old line, flare it properly and install.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer