I have posted many times in my own threads and others threads about clutch problems.
Yesterday I put it in at the good brake & clutch place in town for a bit of diagnostic repair.
Long story short- I've replaced more master and slave cylinders than I can recall, three masters in the last 14 months!

It turns out that the fork and internal movement all feels good to the expert but the hose was breaking down. Crap was blocking the hose making the master work very hard to disengage the clutch and restricting flow of fluid back up the hose causing air to be sucked in. Also crap from the hose was causing damage in the master and slave.

So, replace the hose next time you have trouble.

Steve.