My trucks clutch will not disengage if I hold it in more the a few seconds.
The hydraulic fluid is full. Would this be my master or slave cylinder?

 

The hydraulic clutch system is replaced as a whole unit. South Bend clutch has them. When mine went bad, it was the master cylinder.

 

I just did mine about a week ago, I was having the same problem you are describing. Slave/ master cylinder system. I went with South Bend, it has a braided stainless steel line (not plastic), came pre-bled, the hardest part was getting to old stock crap out with out breaking the line and leaking the brake fluid every where, and the top bolt on the master cylinder. The cylinders are noticibly larger than the stock. I did a few searches to see if I could find anything else out, and read a few times that the dealer part was more money. The south bend runs about $300 US

 

Thanks for all of your help. I'll go check out South Bend right now.