Making erasers nix the white edges.

hiya, I'm using PS7 for this.

Im using erasers to eat away at the edges, but when i make my own brushes (to use as erasers) tiny little white pieces are left on the rim.
like this:

i figured it was because i hadn't deleted the white background before defining the brush preset, and once I'd done that it got a little better (to what it is now) but doesn't go away completely. Can someone who knows brushes point out how Im screwing up?