I haven’t ever come across a vehicle with a sheath on a brake line near a drum or caliper. It might be a factory thing or could be something someone put on there. Does it move at all? Slide up and down or is it fused to the line? If it’s stuck then just take it off with the old line and forget about it. If it moves and you feel you wanna try and reuse it then slide it off once you get the fitting off and out it over the new one. Now as I said if the heat and cold trick with canned air (be sure to hold the can upside down and if possible use the red straw) then best bet is cut the line to get it off.ruining the fitting isn’t the end all be all you can, if you have the tools and enough line left you could re-flare it with a double glare tool AFTER sliding a new nut onto the tube. Youll most likely have to do this with a new piece anyway although some do come with it on there and already flared so uts a matter of screwing it in and connecting to the rubber portion. As for the tool for flare nuts….ive found using a combo of wrenches as well as crow’s feet that work with a ratchet to be helpful. You gotta be sure you are putting pressure straight on though and you are most likely right the damage has already been done so vise grips. Don’t be afraid to crank em down as the fitting is toast.