Before you go striping apart your master cylinder, possibly wasting time and money, reconsider carefully. The ZZR600 calipers are vulnerable to corrosion. The dust seals are likely to fill with green corrosive gunge and this will make the pistons stick. Even if they were seen to move in and out after cleaning, did you carefully check the state of the dust seals by eyeball and in a good light? If the pistons tend to pull the seals outwards with distortion when you pump the pistons outwards, then you need a caliper strip/rebuild and new seals, both dust and oil. You should be able to check the movement of the master cylinder piston by a visual check. I would say that if this was faulty, it would most likely be worn seals or piston bore, which would result in spongy braking, even with no air in the system.