Clutch question for 1985 chevy s10 pickup?

truck seems to shift ok most of the time. occionaly if clutch is depressed to long it acts similar to a brake with no hydraulic fluid...while still depressed it's as if the clutch has been disengaged and the truck starts to move. this can also happen in reverse...if I turn off the engine and pump the clutch, this typically "clears" it up for while. Again, this typically occurs when the clutch is depress for length of time. What's going on?