if it really is brass, good luck. If you can cut the tube and peel it off the lugs then it's a lot easier. Otherwise you have to reflow the brass and separate. Cut away the tube fairly close to the lugs but with enough material that you can still grab it with a pair of pliers. Emphasize heating the tube over heating the lug. You don't really need to get everything hot like you do when brazing -- your main goal is to get the filler to reflow. If you can get the tube slotted to the end, then it will curl away from the lug fairly easily and you can do it in stages. I have used a dremel with a cutoff saw to do that, you have to be careful not to cut the lug, but it's not the end of the world if you do.

Note that the tube may be pinned or even spot welded to the lug. You have to determine if this is the case or you'll never get it loose.