My son and I are fighting a problem with his bone stock Z28. Under high revs(4000 rpm and higher) when the clutch is depressed it will not come back up on it's own. We have replaced the master cyl and bled the clutch three times, the last time to replace the fluid with racing clutch fluid. At lower rpm's the clutch seems to work fine.

Already did a search on site and found nothing. Any suggestions would be apreciated.