Check the residuel valve. Most disc systems use a 1-3 lb valve for disc systems and some do not use any. Put in the lighest valve, if that doesn't do it then remove it alltogether. Check though to make sure after doing this you do not lose the pedal. Apart from that as others here have mentioned, be sure the pistons will retract fairly easy with the bleed screw open. if not then you need to disassemble/clean and rebuild your calipers.