Try when you flare the line not to completely crush the flare on the last step. In the double flare if you leave a small space between the layers then that space can be used to conform to irregularities in the seat and tube nut at final assembly.
Also, I might not tighten, loosen, and retighten the line. You want the tube flare to form to irregularities in the seat, do that once without a chance to shift and then trying to form yet again in a slightly new position.
Old brass brake fittings can be troublesome, inspect the seats for damage and replace or figure a way to recut the seat.
