Most likely its not a problem with any of the checkballs in the valve body, the pressure that an auto trany makes would move the check balls of the seats, it defenitly sounds like a problem with the modulator valve aka:throotle valve. You have all your gears you just cant up shift wich is what the modulator does. Is it a full manual auto or auto with a manual valve? I would check at the modulator ,if you have one, make sure he vacume line is hooked up/hooked up to the right vacume source, also check that you dont have a restriction in the line. if there is no problem with that make sure you have proper mainline pressure. i have a manual that help trouble shoots gm auto trany but i gotta find it, but check the modulator and give me a call after that.