Have you looked at your clutch pedal to see if it is the aluminum one?
Slave cylinder pushrod travel needs to be around 15/16". Do you have this much when you push in the clutch pedal?
A master cylinder can bypass if the seals are bad and still not lose any fluid.
Which engine/trans setup do you have? I had problems with my 85SE v-6 4spd where it would not go into 3rd/4th without a lot of fiddling. It was the shift lever where the cable attaches to the transmission. A pin had worn out and caused the sloppiness. I managed to find a new lever (no longer available from GM) but there is a fix for bad ones if needed.
Will it shift ok when the engine is not running?