That sounds exactly like what it is--mine did the same thing. I actually went for months (a bad idea with hindsight) just topping the reservoir off every few days, and wouldn't you know the slave cylinder decided to give out at around 11:00 pm on a Friday night in the middle of traffic at a red light--the jeep just started lurching forward. My advice, when you can get a chance, go ahead and put in a new slave cylinder and clutch while you're at it (you don't want to pull that transmission twice), so that the same doesn't happen to you! Parts aren't incredibly bad--around $250-300 I believe, but the manual labor in pulling the trans and TC is a b*tch.
Currently an '86 CJ-7---following a string of '01 Sahara, '90 Sahara, and '85 CJ-7. Finally got back to how it all began....