I used the nickel/copper lines on my 47GMC. Purchased a 25 foot roll and did all my own DOUBLE FLAIRS. As you, I has a few leakers. To seal it up I loosened the fitting wiggled the line around a bit and re-tightened. This reseated the double flair. The fitting will take quite a bit of torque. With a back-up wrench and a flair nut wrench I put a pretty good arm on it. Never stripped a fitting. Do not used any Teflon tape. The brake fluid on the threads helped as a lube to get a little more torque. None have leaked since.
