I bought my 1991 s10 blazer 4.3 a year ago. The clutch went out immediately so I got it replaced as well as the master and slave cylinder.
A couple weeks ago my clutch lost pressure and went to the floor so I got the master and slave cylinder replaced AGAIN. Luckily the parts were under warranty.

After I got the truck back I was looking underneath to check things out and noticed a large hole in the front bottom of the transmission near the crank case on the passenger side. It is about 1" in diameter and threaded as if a plug were supposed to go there. I think I can see the metal rod from the slave cylinder when I shine a light inside. Is this a inspection hole? Did the mechanic just leave out the plug? it supposed to be filled with fluid? Thanks for any responses

 

Only thing that requires fluid on a manual transmission is the gear box, which requires gear oil. And if your clutch is hydraulic, that requires hydraulic fluid in it too.