Motorcycle Repair: cleaning brake caliber pistons, emery cloth, ring groove

Question
The pistons are not pitted but have some mild buildup or a dark area at the o-ring.  What is the best way top clean the piston.  emery cloth? scotchbrite pad? or?

Answer
Scotchbrite would be better than emery since it won't scratch the surface.  Also pull out the o-ring and clean the o-ring groove good.  I usually take a bent nail and carefully scrape the varnish like stuff out of the groove.  Just be careful not to marr the aluminum surface.    

Regards
Rich