One of the great things about traditional centerpulls and sidepulls is that the central pivot bolt allows the brakes to track a bent rear rim. If the centerpull arch is touching the frame, however, when your rear rim gets bent, you will likely scrape off a great deal of paint or otherwise bash the seatstays as the brake tracks the bent rear rim.
Here's an ebay photo of a weinmann brakeset with a circular aluminum spacer on the back brake.
http://i.ebayimg.com/
Here's an example of an M6 spacer for brakes :
http://www.biketoolsetc.com/
- Don Gillies
San Diego, CA, USA