If the slave cylinder isn't leaking externally, then odds are it's good. Sounds to me like you have a bad master cylinder and/or you're getting air in the system (in which case you'd probably see a leak). So if you don't see any leaks, then I'd be willing to bet that you have a bad master cylinder.
I had the same problem about 10 years ago with my '85 F150. The clutch would still release, but the pedal didn't 'catch' until it was nearly on the floor- and sometimes it was hard to get into gear. At the time, most parts stores would only sell me a pre-filled master cylinder, line, and slave cylinder as an assembly- for $200 or so. I finally found that Checker Auto Parts sold a master cylinder separately- and they made a big deal of pointing out that there was NO WARRANTY on the thing- I guess they'd had problems with people buying them and returning them when they couldn't figure out how to bleed the system. Cost about $80 I think. It was aluminum, and looked quite different from the plastic OEM master cylinder. But I bolted it on, and it still works fine 10 years later. I don't think I've even changed the fluid in it since.
BTW- when I installed the thing, I tried to bleed the system the same way you'd bleed brakes. It kinda worked- enough that I could drive the truck- but I could tell that there was still air in the system and the slave cylinder wasn't being pushed as far as it should have. I drove it anyway, and the air worked itself out within two or three days. My brother told me that Volvo heavy trucks had similar problems when you replaced a component in the hydraulic clutch system, and their solution was to rig up a setup to pressure-bleed the system backwards... pushing brake fluid INTO the bleeder and up through the master cylinder.