The one problem/deficiency with the design of these disc brakes, is that unlike most modern designs, there are no seals to prevent ingress of moisture or dirt. The O-ring (actually rectangular instead of circular in cross-section) is there to provide the piston pull-back and to isolate (ok seal) the hydraulic brake fluid chamber. As previously described, any interference at this seal will cause it to hang-up, and the only aid to keeping it clean is to try and protect it with a bit of grease - which naturally has to be able to withstand very high temperature.
If you carefully look at the groove in the piston wall of the caliper, where the O-ring sits, you will see that one side is "rounded" and not a square shoulder. This allows the O-ring to "flex" and roll when the piston is forced outwards under brake application, and then roll back as it pulls the piston in when the brake is released. If there is any dirt or material hampering this action of "rolling back", then you have a dragging brake.
