I've searached like crazy here and elsewhere and my problem seems similar but different than all the other "locked up clutch pedal" posts except for one from 2006 which got zero replies.
My '95 YJ (4 banger, NP231) has a recurring problem where I'll be driving along, then the clutch pedal won't engage. It's hard locked, as though there's a brick under the pedal. No matter how hard you hit it, nothing, no movement. If you verrrrry slowly engage it, it will usually go down, and I've adapted a strange driving style for when this happens, for safety reasons (pop it in neutral, and start gently feathering the pedal until it eventually goes in, and drive on until I can get home and do the 'repair' - see below).
I've read a number of opinions about this, and have checked physical linkage from pedal to clutch cylinder and all seems good, fluids good and full, no leaks or bubbles or anything, I even bled it out for good measure a couple of times, but here's the weird part: It'll be fine for many months at a time. Last time I had the problem was last July (2011), and I'd read that the hydraulic clutch could lock up this way when air got in the line from temperature or humidity fluctuations (vaaaguely plausible to me), the solution being to pump the pedal (a lot) with engine off, and I sat there and pumped the pedal for a good 15 minutes, and voila, it worked fine again, for almost a year. It worked fine until last week, when I was driving along and almost rear-ended a bus as it just suddenly locked up after showing no signs at all of acting up.
After getting home, I did 15 minutes of pumping the pedal with cold engine and voila, it works again, and I won't be surprised if this works fine for another 8+ months.
P.S. It's done this for the 17 years I've owned it, started when YJ was about 5 years old, and about once a year, this happens. I've had the clutch replaced once in its life, about 9 years ago (This is not a DD so that was only about 4,500 miles ago). I have no rough shifts, my clutch engages normally 360 days a year. I have no clunks, no other symptoms whatsoever. Fluids good, master and slave were replaced along with the clutch, and I'm stumped.
Anybody have similar experiences or advice? It's more of a nag than anything, as I said, it's perfect 360 days out of the year, but seems so weird that it just keeps happening about once a year or so, roughly, and with such an 'easy' fix (if you can call it that.. maybe bandaid's a better word).
I can't help but think it's mechanical somehow because it really does not feel 'squishy' or like a hydro-lock. It is like there is a solid brick under the pedal and it will not move even 1/4" (other than bushing play)