2011 1.8 clutch issue

Hi all,

A friend has a 2011 octavia 1.8 tsi with a clutch issue. Not being too familiar with vag i wanted to know what work is involved. 

He has been told its the clutch and dmf at fault and suggested that i had a look. 

Taken a look, the brake fluid is a very dark green almost black, no obvious signs of a leak anywhere. 

Upon driving the car, the fault quickly made itself apparent. Clutch pedal will not always rise back to its original resting position, stopping half way without fully returning. 

Setting off from a stand still proved no slipping clutch and accelerated cleanly away. To me the issue appears to be hydraulic related. When i drove the car and noticed the clutch pedal not fully returning i lightly put my foot on the clutch and quickly flicked my foot off and on the pedal without disengaging the clutch and it suddenly came back to its normal resting position. Its almost as if something i.e the piston inside the master cylinder gets stuck and pulling the pedal up or slight pressure applied,frees it off. 

What are your thoughts? I dont believe it to be clutch or dmf related myself. Is it a difficult job to change the master cylinder? 

Thanks in advance!