Agree with Mike.
Check the fluid level first. Top culprit.
Having a shared reservoir for the brakes and clutch sets you up for this. As the brake pads get low, the fluid level drops, and the clutch stops working. The clutch feeds from higher in the reservoir, so the brakes continue to work.
Otherwise it is almost certainly the slave.
Slaves have a much higher failure rate than masters, in my experience.
Mike's 'gelling' problem was likely the result of frozen water accumulated in the slave. The clutch fluid should be bled clean periodically to prevent this and other related failures.
This is a hydraulic problem.
Definately not the pressure plate or T/O bearing, and there is no cable involved.
1986 300E 5-Speed 240k mi.