Vacuum Check Valve
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2003 Volkswagen EuroVan, 2002 Volkswagen EuroVan, 2001 Volkswagen EuroVan, 2000 Volkswagen EuroVan, and 1999 Volkswagen EuroVan. 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. See Fig 1 . Valve must remain closed in opposite direction. Ensure that check valve is installed correctly.