Mercedes sprinter clutch issue?

Hi I have a 2010 Merc sprinter 313cdi just recently having an issue with clutch pedal becoming very easy to push in in but afder driving for a while it goes back to normal with the compression back I thought it might be the clutch but done the usual checks and clutch even tho the bites high is still there no weird noises knocking judders I have just been to a garage and he has quoted me £900 for supply and fit a new lux full clutch kit but I explained to another garage he thinks it's the master cylinder is letting in air that's why it's so easy to push in and when driving for a while it's getting compression as it's getting rid of the air as a I drive any help would be great thanks mark