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[Garbage_Mechanic]

Roboto has some good points but secondary to the real problem. If the pedal sinks under constant pressure, the hydraulic system is bypassing. Also, this system is designed to have as close to ZERO free play at the pedal as possible. This is backwards to all the years prior to 83, where if you didn't have free play you were in trouble. Also the clutch hyd system is no different than brakes, what would you think if your brake pedal sank to the floor with your foot sitting on it. I think you need to be sure the hydraulics are good and there is no more than 1/32 inch free play in the master cylinder pushrod, and that the cluth pedal shaft bushings don't have excess play (read as none).
As to lubing things up....while you have the slave cylinder off slip the fork out of the bellhousing, comes right out, just pull and wiggle. While its out you can use a spray can of penatrating lube to lube the slider where the throwout bearing slides. Its common for this to get gummy as Roboto said, but you are dealing with dried up dirt and grease mix. Also lube all of the pivot points on the fork.

After all of this, if it still doesn't release properly, you will still have to pull the clutch. I'm going through the exact thing right now on my ranch truck. The release problem on it was dirt and worn off clutch material between the diaphragm and cover. I couldn't justify the $600 smfw either although I would love the decreased pedal pressure etc. Good luck, let us know how it goes!