One problem related to brake disc pads has been reported for the 1999 Audi A6. The most recently reported issues are listed below.
Two vehicles in front of my car on highway came to a stop; I applied brakes, pressing all the way to the floor of the car, and nothing happened - the car did not slow down and the abs did not engage, despite there being loose sand and salt on the road surface. I rear-ended the vehicles resulting in substantial damage to my car (over $10,000 to repair) and the car I hit (unknown cost covered by my insurance company). My car spent 2 months in the shop having the accident damage repaired. When I picked it up, I tested the brakes and they were very "soft" - I had to press the brake pedal all the way to the floor to get the car to stop, even though I was going no more than 10 mph. The abs did not engage that time either. I returned the car to the dealer to fix the brakes. They kept it all day and told me that nothing needed to be done. However, when I tested the brakes again, in the same spot I had tested them before, the brakes worked well and the abs engaged. On October 31, 2003, the dealership (bernardi Audi, 521 worcester road, natick MA; 508-651-9980) serviced the vehicle and supposedly replaced the font and rear brake pads and rotors.