Two dissimilar metals can react to each other, maybe that is what is going on, I have no idea if brass and stainless react though, that is a question for the chemists in here.

What I do know is this, brass is used in alot of underground water service applications because it is pretty stable and will last literally decades. And most of the reasons for watermain breaks are from electrolosis, where different metals or minerals react to the metal, I have seen the bolts completely rotted out of fittings, and all you had to do to fix the leak was replace the bolts and retighten.

Basically if brass was in there I would definately replace it with brass. I have no idea where you would find it though, I would imagine that Ace Hardware would have them, they have thos little boxes with all kinds of odd stuff in there, Home Depot has one of the saddest nut and bolt selection in the world, as does Menards. Lowes is not bad, but Ace never lets me down.