The bolt slides into a metal sleeve with a boot around it. It allows the caliper to move freely while the pistons push against the pads. If you add a washer or over tighten it (like I did) the caliper will not move freely and the pads will drag on the rotor. The bracket is made to bottom out the bolt and prevent this from happening. I guess I don't know my own strength. I hope my mistakes will help others.
