Ok a couple tricks…well first I HAVE to ask are you using flare nut wrenches or crow’s feet? Because normal creacent wrenches just round off line fittings.
That out the way, a little heat can help ive also tried another trick using canned air. Turn it upside down shake the hell out of it and spray while upside down and it acts to freeze the metal a little bit. Sometimes that works alone or used after using heat. Just be careful wear safety glasses. I live in NH so deal with a lot of the same issues and the question I often ask myself it- will trying to save this rusty line cost me more time and aggravation than just getting it off and replacing with a new one? Seems either way youll have to do a bleed on the system, most go for stainless but really stainless rusts as well just not quite as fast. SO if you can’t get it off, takes too long etc, cut the line so you can get an extractor socket over it, tap it on and itll come off pretty quick. Its a small section of line and if you are replacing the drum cylinder and all that its really is easier to replace the aging lines around it. Just be sure to bleed properly. Did brakes on my brother’s dodge where someone wrenched off most of a bleeder nipple so they just didn’t bother doing that 1. Caused a bubble in the line allowing moisture to collect and it rusted from the inside blew out in my driveway. The nipple was so bad even with extractor sockets and drilling and easy outs, heat all of it, just wouldn’t budge so had to replace the caliper. While I was at it I replaced all of his rear brake lines as well.
If you are really set on trying to save the tired old lines try the heat and freeze trick just b sure to use flare nut wrenches. If it won’t come off that way (even if you are using vise grips at that point) then get a good set of extractor sockets (Amazon about 25 bucks or so) and that WILL get it off.