My trans started acting like there was air in the hydraulics. I'd push in the clutch and it wouldn't go into gear without grinding. So I replaced the master and slave cylinders with the GM replacement and it's still doing it. I jacked up the rear and with the engine running and the clutch depressed just pushing the shifter towards any gear makes the tires spin.

It seems like the trans input shaft is binding in the pilot bushing or something. Any thoughts?

 

If you replace the master and the slave there is most likely air trapped in the system. To bleed it remove the cap on the reservoir, then unbolt the slave cylinder and hold it diagonally with the line at the highest point. Now slowly push the lunger in by hand and let it return to it's normal resting place. Repeat about ten times and you should be good to go.