I recently put a thread up about our A6 being abit rubbish on brakes and I thought it was the servo.
Well unfortunatly it wasn't. I found this by replacing it and it not fixing it.
It turned out to be the press fit on the vacuum outlet of the vacuum pump being loose and letting air in.
Took it off, sealed it up and pressed the fit abit tighter.
Brakes are 100% better now.
Also fitted the 312mm kit I took off my A4 at the same time.