I have a '92 k2500 with a 350 and the nv4500 and i've been having some trouble with the clutch lately. Every once in a while,usually when its warm, the clutch pedal feel just goes wack. About the last 1/4 or so of travel the clutch pedal feels kinda jittery like something isn't lubed up like it should be. Then when you let it out its hard to take off smooth without the truck lurching like crazy. I sprayed wd40 on the pivot point on the pedal where the spring is. This made the clutch feel a little smoother overall but it still has the bad spells just like before. I don't know too much about the internals of clutches but i know it aint supposed to be doing this. It never slips or anything its just that the padal feels crappy. So whats the problem? I'm suspecting the master cylinder or some part of the clutch slave but i don't really know much about hydraulic clutches. Also if anyone has a link to a good writeup on this clutch or maybe some pictures of how everything in there works that would be AWESOME!

The pedal feels all jittery

I am willing to be the throwout bearing is going bad or the fingers on the clutch pressure plate are traveling out of square due to a bad pivot ring in the pressure plate. you can inspect the travel from under the truck by removng the cover on the left lower bell housing. 3 10mm headed bolts and it pops right off. I do not need you getting killed because your head got squished. please block the tires and applie the parking brake.
as you watch the clutch operation from the side you will see the end of the clutch lever. its attached to the throwout bearing at the other end. you may beable to use a mirror while the engine is off to veiw up inside the bell housing.look to where the throwout bearing is and see if the fingers touch all the way around.

if you try and veiw the operation with the engine running be VERY careful. and please don't stick anything near the clutch if its moving.

the presure plate is most likely the problem, and would need replaced if you see to much wiggle in the lever.