How do you know there are no leaks? If it's a tiny airlock, then it's probably an unnoticable amount of fluid from a simple visual check. Try going around all the joins with some clean, dry towel/paper - it will quickly wick any fluid and show you the tell tale of a damp spot on the paper - check joins through the side skins, 3-way joint, and into the master cylinder. I had a similar nightmare with mine - it was fine on road, but any track use and I'd have a problem. Eventually cured with new calipers (not that expensive from rimmerbros). I think the bleed nipples themselves weren't actually sealing properly although I also had an issue after replacing the master cylinder where the old pipe flare didn't seal properly to the new cylinder. I made up a new section of pipe with a join into the old pipe that solved that.