Constantly Variable Clutch Pressure

[TahoEagle]

I've owned my '85 Eagle for about a year and a half now, and it's always had (what I presumed to be) a leaky master and/or slave clutch cylinder (5-speed manual). Leave it sitting overnight and there'd be no clutch pressure to speak of in the morning, and a nice little puddle of fluid on the driveway. For the longest time I'd just dealt with it by filling up the reservoir tube with DOT-3 brake fluid and pumping the clutch until I'd build up pressure and be on my way. Well I finally bit the bullet to get the slave & master clutch replaced a few weeks ago. All seemed to be going well, until the mechanic called me up to tell me that, although the new slave & master cylinders had fixed the leaking problem, he still couldn't get the clutch pressure up, even after bleeding the system twice. When I went to the shop to take a look at it with him, the pressure had somehow built back up again, and I was able to drive off no problem. Things worked well for a few days, but then the pressure problem returned. A glance under the hood showed that reservoir tube as empty, but no major puddles on the ground. So I'm once again doing the DOT-3 routine, but now clutch pressure is more variable then ever. Whereas before you could build it up after pumping for a while, now you could build it up and it'd be gone shortly thereafter only to return in full force a few minutes later, then disappear again. I'm noticing much fewer puddles on the driveway now, but am still thinking there's a leak somewhere. Is there a location in there aside from the slave & master cylinders where leakage could occur? Could that  reservoir tube be the culprit, or could there be something more serious at play? The previous owner had told me that she'd had the clutch replaced less than a year before I bought the car as well (but, of course, didn't keep any service receipts)