I don't know how things in SC are for rust but being brought up wrenching on MN cars I've dealt with my share of rusted brake line fittings.
If you plan to replace the hard brake line the best thing to do is CUT the hard line close enough to the fitting to get a 6-point socket on it. Then try with the socket. DON'T GET TOO CARRIED AWAY WITH IT AT THIS POINT! If it still won't come off get out a propane torch and heat it up. MOST of the time you can re-use the fitting if it is in good shape. The replacement line at the auto parts store doesn't always have the right fittings attached to it.
Since you are going through all this to fix that one fitting, if the replacement line doesn't have the correct fitting I have found a good selection of the correct fittings at NAPA.