Hello Mr C
I have had the same problem, with my oversized Slave Cylinder, I drained the clutch fluid, topped up the reservoir then took the cylinder off and pushed the piston back into the cylinder, be careful as air/bubbles will work there way back into the fluid reservoir I think this is called "back bleeding" then pumped the lever with the bleeder valve on the banjo bolt slightly undone, topping up as you go.
Saying all that I noticed that once all tightened up when the lever was pulled I could hear a hissing noise coming from the cylinder, which would suggest air is getting out/in.
I think its coming from the cylinder onto the crank case, not sure yet as it got to cold last night. But seems odd to have a compressed cylinder with no seals onto the crank case. (metal on metal so to speak)
I replaced the pushrod for the longer one.
When I know more will let you know. Sorry not much help I know, but it must be the cylinder end air is getting in
regards
TopiToo