Valves, Checking
WARNING: This page is about a different car, the 2018 Audi A6 Quattro, 2017 Audi A6 Quattro, and 2016 Audi A6 Quattro. However, it is still accessible from the selected car via links, so may be relevant.
-- Check valves at stem and seating surface for traces of wear.
-- If there are clear traces of wear, replace valve.