[Tom Goldman]

The idea behind a square o-ring is that it will rock slightly when pressure is applied and rock back when released , thereby slightly retracting the piston.
If you are experiancing excessive drag ,I'd be looking at caliper squareness, or excessive residual pressure caused by the brake pedal /master cyl clearance or the master cyl itself.
Calipers not being parallel to the rotor is a common problem ,especially on rear housings.
Even when using a good narrowing fixture ,I've seen housing ends measure .015-.020" out of square at the housing which is .060" or more at the end of the rotor.