Vacuum Check Valve
WARNING: This page does not describe the selected car, but rather 15 other vehicles, including the 2005 Audi allroad, 2004 Audi allroad, 2004 Audi A6, 2003 Audi allroad, and 2003 Audi S6. However, it is still accessible from the selected car via links, so may be relevant.
To check function of check valve, blow air through valve. Air must pass through in direction of arrow. Valve must remain closed in opposite direction. Ensure that check valve is installed correctly.