If there's no fluid leaking out from the flex line joins when you press the pedal its pretty unlikely that air is being sucked in on the return stroke...
The design of the slave means that, when bleeding, you can have clean fluid coming out the bleed niple but a big air pocket in front of the piston - and consequently, no clutch movement. I suspect that is why Islandblue stands the slave at an upright angle (correct??) when bleeding.
Another approach (that I use) is to uncouple the slave rod and physically push the slave piston back to the bleeder end of the slave (open the bleed valve when pushing the piston in). I then hold the piston back while pressurising the system using a Gunston Ezibleed at a couple of psi air pressure. Once all the air is expelled, reconnect the slave rod and then you will need several pumps of the pedal to push the piston back to its operating point.
Another possibility (BTDT) is to concentrate so hard on getting the air out that you forget to check the level of fluid in the master cyl - at some point you run out of fluid and start pumping air (rather than fluid) into the system
The only other expanantions that I can think of are:
a. the master cyl bore is damaged (pitted??)
b. you installed the wrong master cyl seal kit or
c. the kit was not correctly installed...
But, knowing its a Mini, I would never (sic) say that there is not some other explanation....