Very old brake lines tend to seize. You need to soak the connection with good penetrating oil like PB Blaster or a 50/50 mix of ATF and acetone (which works better than PB Blaster). WD-40 won't cut it so don't waste your time.
Drench the heck out of the connection twice a day for a week straight. Once in the morning, once in the evening.
While that's soaking, go and talk to your local Snap On guy about getting some good flare nut wrenches. The hardware store wrenches are worthless. Though if you've already rounded off your flare nut, just go straight to the vice grips. Get them good and solidly clamped on the flare nut and it should come out.
Even so, with very old hard line rust may have settled in and you may end up twisting the entire end off the hard line. I did that last time I swapped out brake lines. I had to build an entirely new hard line. But it was the drivers front so it was short and easy to replace.
