I went through 3 slave cylinders and 2 master cylinders before I found and fixed the cause of my problem on my '02 Ranger. I had almost the excact same situation with the clutch pedal. The cause was the flywheel. With the slave cylinder being mounted to the front of the tranny, if someone machined the flywheel and didn't shim either the flywheel or the slave cylinder the same amount, the slave cylinder has to travel farther to do it's job. When it travels too far the piston goes slightly past one of the internal o-rings allowing the fluid pressure to drop off. You won't have a leak that you can see, the hydraulic system simply can't do it's job! I wasn't happy doing the R & R on the tranny and transfer case over and over until I found a knowldgable Ford mechanic. I replaced the flywheel, entire clutch assembly, and slave cylinder all at the same time, problem solved!