Thanks for all your thoughts and advice, I suspect their may be a problem with the master cylinder or the pipe that connect it to the reservoir. Only because, when the piston is pushed back into the calliper there's pressure and this doesn't rely on the cylinder to generate the pressure.

Thanks for the offer of the loan of the ZZR cylinder it looks the same as mine. If you want to PM me your address and are about Saturday afternoon I could call in a pick it up. Cheers John