If the cylinder seal (either o-ring or metal rings) is leaking the lift will not stay up after the cluth pedal is depressed.
As others have suggested you should pull the inspection cover and see what is going on. My guess is that you have sticky valves in the pump chambers. I have essentially the same problem with mine and I have traced the problem there.
I am too cheap to pull the pump and rebuild it so I am just putting up with the intermittent performance issues.