Eventually had time to play last night, turns out the fluid in the whole system was so dirty it was untrue. The sediment was actually blocking the feed from the res to the cylinder itself so as you pumped it no fluid was being drawn in to replace it. I ended up stripping the master cylinder down completely to clean it out. now it is pumping fresh fluid I've started bleeding it. It bled fine to the nipple on the flow valve. The rear brake did seem to be taking ages to get fluid but what I did notice was that if I pumped the pedal a few times then released it I got a nice steady stream of bubbles in the res. If I pumped it about 10 times I even got a nice little fountain of fluid breaking through the surface of the fluid in the res. Anyway, hopefully tonight I'll get it finished.