Did the same this spring on my 5-ton. Priority always is saving your nuts (
) UNLESS you have a complete replacement already laying in front of you AND are sure it fits.
First trick: day before clean and put WD-40 on top of it, few times.
For the rest, Bob gave good advice. My own experience, an open-end wrench ALMOST ALWAYS slips on these @#$%$#@ nuts and start rounding them. Buy a special brake-line wrench (or whatever the name for that is). That is a wrench which is almost as a 6-corner box-end wrench, but with a small opening just fitting around the brake line. Those wrenches give you max grip and mostly that works. Just googled-wiki-ed, those things are called - big surprise - flare nut wrenches.
Make sure you have the T-split in a good grip, and then try apply the force in a short burst, maybe hit it with a hammer a few times first (we called that "scaring a nut"). Slowely increasing power mostly results in quicker slipping.
If that fails, just cut the brake line and use a good-fitting socket wrench.
The vice-grip is my last resource, mostly you can throw the nut away. No prob in the USA for MVs, a disaster in the Netherlands because we do not have replacement brake-line nuts with tthe same thread here. That is a good incentive for being very carefully