Citroen DS3 clutch pedal sticking to floor

Hi, i have the same problem with a customer and im wondering did anyone get to the bottom of it. I Fitted a new genuine master cylinder 2 and a half months ago and it bled up and drove fine until the same thing happend again. i am the 3rd person to do this but i didnt trust the previous repairs. (2nd hand Parts) when trying to bleed, the pressure seems to be going into the resovir. I'm thinking the servo but would like to know if any1 else has cured this problem. Thanks