I have a 94 Honda Civic DX, the cluch seems to work fine until I drive for 30-40 min. Then it go's to the floor and is hard to shift without grinding gears. I have checked the master and is full of fluid. When it sits for a few hours it seems fine again. Could this be the master cylinder or cluch plate ?

. Change the clutch fluid (DOT 3 or 4 brake fluid). Ensure all air is purged from the system.

. Change the clutch slave cylinder, if the problem continues:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-2.html#post232189

It could be the clutch master cylinder needs changing, but it's more likely to be the slave cylinder.

Yes I agree. Clutch needs adjustment or replacement. Does the synchronizers worn?