Just got pointed to this topic. I used a little brass brush to clean up the pins and caliper pad area. I also run a fine file flatly across the piston contact surface to be sure no trash is stuck to it that would cock the pad. I just had a lesson in what happens if the piston is not rotated to its proper 20 deg position. Ruined a good set of porterfield R4S pads!
-Tom P.