if you put your foot on your brake pedal really lightly will it eventully go to the floor? if then there is internal leakage. the front pads being low, the pistons should still be able to come out a fair bit more. though the pads could be overheating the brake fluid. and causing them to stick. Or last possibility is that, although you said the pistons slide in and out ok, the rubber seals in the calipers are not pulling back the piston enough.

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Version 6 STI