I just installed a new master cylinder, flexible brake lines and rebuilt the brake cylinders. Hoped to reuse the existing steel lines, they looked good from the outside. DEFINITELY use flare nut wrenches, don't EVEN try an ordinary open end wrench on these 50 year old fittings. All but one came off relatively easily after soaking each joint for a few days prior in Liquid Wrench. But that last one was a BEAR, connecting to the flex line inside the RF wheelwell. Soaked for over a week in LW. Propane torch for heat, alternated with aerosol air duster to freeze, used the special vice grip in the third photo above (only the smallest size 4 fits), NOTHING worked. All wrenches just spun on the nut. After a week of frustration, I resigned myself to replacing that line since I knew the fitting would get destroyed during removal. I took regular serrated curved vice grips, clamped HARD, froze the CRAP out of it and tugged. POP! She broke loose! Amazingly the nut was not mangled badly. I cleaned up the threads and test fit it to the new flex line. A flare nut wrench still fit perfectly and I snugged her down. Got REAL lucky, zero leaks.