If the nut is rusted on. Just changed a clutch line in the silver turbo today (same 10 mm flare nut as the brake hardline) and all three of the 10mm flare nut wrenches I had spread open and started rounding the nut. Broke it loose with a large crescent wrench. Tighten the crescent wrench on the nut, turn it... If you see/feel it slip, back off and tighten more and turn again. Took 3 rounds of this to break loose a flare nut that was installed in 1990. Flare wrenches and regular 10 mm wrenches were useless for the initial cracking loose... but worked fine after that. Vise grips may work as well, crescent was the trick today for me.