Large ID on washer so that if you install on bare brake-boss, it falls all the way to the bottom. The standard washer would sit at the end of the boss and be pinched in place by the bolt.
Some people grind/file the end of the boss to shorten it to remove the play. But it's very easy to overdo it and end up with a post that's too short and the bolt ends up locking the brake in place with no movement at all.
Standard washer looks like this:
http://www.avdw32.dsl.pipex.com/imag...washer-8mm.jpg
The washer I'm talking about looks like this, about 0.1-0.3mm thick:
http://www.americanmodeleng.com/site.../washer.01.jpg
