iv,e just sent my cube back to tredz, because of this issue, mine are m615,s, the rear brake was never as good as the front, no matter how many times i bled it, it would randomly pull back to the bars, but when pumped it would be solid again, i think its a shimano qc problem, its about time they started selling spares, instead of people sending back their brakes under warranty, i would of bought either new seals or diaphram for a few quid, rather than be without my bike for a few weeks, while shimano look into my rear brake. i couldnt find any leaks either, and when the mech at tredz looked at the brake he couldnt understand it either, they have been sent back to madison.we will see what shimano have to say, not expecting them to admit to anything though, they will probably just send a replacment brake back.