i don't think it only does it on an incline,,,,and it does'nt just need an adjusment. you need to just go down to discount auto parts and buy a new clutch slave cylinder for about $13. 2 bolts and unscrew the hose. put the new one on. open the bleeder valve and let it run until no more air(tap on it while running), and also make sure there is enough fluid in the reservoir before and after doing this. have everything you need right there so that you don't have to clamp the line, just put your finger over it.
Why are you so sure it's not the Grebner valve? Go to the clutch master cylinder. Now loosten the nut on the right side, the one that doesn't seem to do anything. Turn it until the slots line up. It shouldn't leak but if it does it's ok. If you push the clutch pedal now you should hear a 'fffft, fffft, fffft' sound. If you don't, turn it till the slots line up again. Ok, now tighten it back up again and fill up the master cylinder. If you get too many bubbles, you should only pump it while the tap is open.