Cluch problem 300tdi

I have recently changed the clutch fluid on my 300tdi disco as the old fluid had past it's time. About 4 weeks later my clutch pedal dropped to the floor and the fluid was on the min mark. Topped it up and bled the system and the pedal returned to the top. I test drove it and around town it's fine. If you don't touch the pedal for around 3-4 minutes the pedal is half wY down to the floor and on the floor in 5 minutes. You can pick it up and pump the pedal and the clutch will return but then the problem repeats itself. I have not lost anymore fluid.

Anyone any ideas as to what my be happening???