I go to town on the brass with Bar Keepers friend. I use a q-tip dipped in it and make a real mess of it. Lol. I use a toothbrush dipped in water to clean up the leather or the zipper material afterwards or as I go along. THEN I dunk it immediately AFTER doing the brass. I sometimes have to use a little polishing cloth after the bag is dry to get the brass really shiny, but doing it this way, I don't have to worry about getting it on the leather.I checked on mine and it is pretty tight too. I wouldn't want to loosen it up by forcing it. You can put vinegar on a q-tip and wiggle it into the parts you can't reach. That won't stain the leather.
I used to do the brass last, and I hated it because the leather would be pristine. it took a lot longer to polish the brass doing it last since I had to be so careful to avoid getting it on the leather.