Best to use a new rubber bushing which attaches to the groove on the end of a manual brake rod. It keeps the rod from popping out of the MC piston (or rattling). If the MC piston jams in the bore (from rust), the rod could conceivably extract enough that you couldn't push the piston again. Since those bushings are hard to find alone, I think some people have cut rubber hose or used an O-ring.
