96 Z24 Cavalier clutch slipping

I have a 1996 Chevy Cavalier Z24 with a manual transmission. A couple of years ago I had a problem where the clutch would not disengage and allow me to shift on my way home. I took it to a mechanic that replaced the clutch for me. About 6 months later, about summer time when the weather started to warm up, the clutch would slip. I took it to a different shop that I trusted and they replaced the clutch again and said it looked like the flywheel had not been resurfaced. It has now been a year later, again once the weather warmed up, and the clutch is doing the same thing. It was suggested by a friend that I needed to adjust the clutch, but this is a hydraulic clutch. It is now slipping really bad and has lots of play in the clutch pedal. Could this be a problem in the Transmission or a problem with the hydraulics? When it is cold out and the car is cold it runs just fine, as the weather warms or you drive the car long enough to really get it warm the clutch starts slipping and will disengage almost immediately when pressing in on the clutch.

When the clutch get to the point you can't shift at all. If have found that the rubber piece on the lid of the clutch fluid is sucked up into it's self. If you pull it out and replace the clutch will work again.

Try replacing the cap. If it's not venting, pressure may be building up and the slave cylinder may be applying a bit. Try bleeding the fluid too.

Thanks for the suggestion, I will give that a try.