I have had luck freeing up frozen connections at the rubber to metal brake line junction by spraying with a good penetrant (PB Blaster or Aero Kroil) over several days and then using the proper sized flare nut tool and at the worse case scenario either using a propane torch with a homebrew needle nozzle directly on the frozen connection nut and then spraying the metal brake line with "Insta Freeze" which breaks the bond between the line and the lock fitting OR use a dremel tool with a carbide wheel and cut across the fiting PARALELL to the line until you are almost through and then split the fitting.

Both methods turn the rubber line and the fitting into trash but most times I was replacing the rubber lines with braided hoses. I have heard some folks claim the heat up method left the fitting still in useable condition...

I guess it depends on how much you use the "blue tipped wrench"!