Did you ever bleed the brakes?
Last I have heard, my old 1990 Coupe Quattro had 160K on it and still had OE master cyl. I assume A4 will last as long.
You don't say if you ever bled the brakes. If so, how did you do it? Standard method of one numnut pumping brakes then holding pedal down while other numnut opens bleed screw puts major strain on MC, since you force the piston seals past their normal travel points (look inside an old MC sometime; you will see a wear pattern due to normal piston movement).
I went thru 2 master cylinders on Porsche 911 in about 18 months back in early '80's due to bleeding this way. Switched to pressure bleeding, never had another problem (incl. 120K on '84 and 50K on '95 Porsche, plus above Coupe Quattro).