Hey I'm having this problem too. The other morning I went out and pushed in my clutch to start the truck and it only came back up a tiny bit from the floor. Anyway, I started it, moved the truck a little bit, and checked the reservior. Looked full except that the rubber plug thing that sits in there was terribly distorted. I just pumped the pedal a bit and it came back. This morning the same thing happened. Kinda seemed like as the truck warmed up the pedal came back, although it doesnt make any sense. And I dont think the pedal is quite where it should be, since the clutch seems to be slipping just a little. Any progress with your truck?
1988 Ford F150 4x4 - 4.9L I6 - 5 spd
2001 Hyundai Accent L - 1.5L I4 - 5 spd