One problem related to power assist brake vacuum has been reported for the 1999 Audi A6. The most recently reported issues are listed below.
Recall # 99v1314000 vw/brake booster; owner received recall notice in September. Dealer was contacted in October/ November and December, 1999. Dealer stated that no parts were available to perform recall repairs. Also, brakes woud malfunction while driving in temperatures of minus 4 degrees. Owner is having vehicle repaired.