I've found a Vise-Grips product called the Locking Wrench (as opposed to their locking pliers) to be an excellent tool for brake line fittings. It has a V cut into the fixed jaw so it can grip the flats of a hex head on three sides and lock onto it without munging up the fitting. The smallest of the three sizes available is the one you want for brake lines.

I used this tool on an old Audi quattro that had seen 15 or so salty winters, and the line fittings had lost some metal to corrosion. Even flare wrenches can slip under those conditions -- but not the locking wrench. Also used it on my dad's farm truck, a '67 Ford with horrendous rust. Worked like a charm.

Russ M.
'82 300td
'82 300cd