One approach is to use stainless balls until the bronze seat is well established, then switch to nylon or Delrin balls. Since the ball is much softer than the seat, you are controlling where the wear will occur. I'm still using the balls with stems that came with my pumps and haven't done this myself.
Dave Nergaard designed a nice O-ring-based check "ball" - [www.steamautomobile.com
I got some of these made and am keeping them in reserve.