losing clutch fluid

I have a '90 4x4 3.0 V6. Recently, the clutch pedal gave out and lost fluid so I changed the slave cylinder. Three days later, the pedal went soft again and I lost the fluid. I have checked up and down for leaks, but can't find any. It seems only to lose fluid when it is driven for more than 15 minutes. No puddles underneath and everything else looks dry including lines, damper, and fittings. I have sat and watched as the pedal is pumped repeatedly, and I see no leaks or even loss of fluid. The system has been bled twice. Can't figure where this fluid is going! Any ideas?

Try the Clutch master, and look inside the car where it mounts to the petal , my guess is the fluid is on the inside not out... GL

That is precisely what I noticed this morning. I will be replacing the M/C. THanks.