Hi for what its worth I had this problem over about two years, multiple master cylinder changes multiple brands of cylinder, finally cracked it as it was all about the fluid used, I was of the opinion that all DOT4 was the same, ITS NOT, finally settled on Comma ESP 4 and a new LUK cylinder along with a complete flush of the braking system with Comma, gravity bled the clutch slave cylinder with a auto fill top up bottle and two years and 80k later no problems with the clutch pedal hitting the floor first thing in the morning, an oil/fluid technician I spoke to said that the cheaper brands of DOT4 contain an acidic compound which causes the seals to degrade and wear and in his opinion the Comma is as close to genuine Audi fluid as you can get.
Steve.