My F250 351ci 4*4 Hydraulic cluch uses fluid. I can not see any leaks. When the fluid reservoir drops, once or twice a week, the petal stays down. It's worst in cold climate. When full of fluid the clutch works fine.
You don't mention the year, but I had a 94 F-250 with the 5 speed manual which went thru 2 slave cylinders (which also serve as the clutch throughout bearing) in 100,000 miles due to leaking. The leaking fluid ends up in the clutch housing. It's a real pain in the rear to change, since the transmission's input shaft passes thru it, requiring the transfer case and transmission to be pulled to get the new one in! The symptom is that the clutch engages closer and closer to the floor, until the clutch barely disengages at all.
Unfortunately, you have to pull the transmission and transfer cases back more than a couple inches from the clutch housing and replace the throwout bearing. Depending on your mileage and driving habits, this might be the time to replace the clutch and possibly the pressure plate as well. The first time it happened, I was short on time, and had to let Ford do it. They didn't recognize the problem (I guess I got a dumb mechanic) and he replaced the master cylinder, clutch and pressure plate as well. No amount of argueing would get me my truck back for less than $1500! Oh well.
The second time it happened, I retired the truck to part time duty as a local work truck at a summer camp, where it probably gets less than 1000 miles a year. Never fixed it, I figured I'd wait and see how bad it gets. So now, it engages about 1/2" off the floor, but otherwise still works well. Hasn't seemed to have gotten any worse in the last year or so. I don't understand that. I would have thought I'd eventually be starting the truck in gear!
If you can find an aftermarket throwout bearing that is a bunch more heavy duty than Ford's original, let me know! I really hesitate to put the same problem back into the rig by using another of Ford's gem.