[murdog94]

Honestly on my 1988 c1500 it was both, and then the line failed. But usually the slave cylinder is the one that fails first.

 

[murdog94]

I would start with the Slave Cylinder.. parts are cheap enough. Since it is allowing air in. Then go from there

 

[murdog94]

basically none because your clutch has pressure at first becuase the air bubbles that are getting into the system they bleed out over night. once you have used it again and gotten air in them then the clutch will work as it wants to. So start with the slave cyl, bleed it really well, and if the problem is still there then your master cyl is gone as well since they tend to go out pretty close to each other with use.
As i said cheap parts, and you dont want to have either fail because driving the truck becomes basically impossible.