WARNING: This page is about a different car, the 2011 Volkswagen CC, 2010 Volkswagen CC, and 2009 Volkswagen CC. However, it is still accessible from the selected car via links, so may be relevant.
The check valve -A- must allow air through in direction of -arrow-.
Fig 1: Checking Check-Valve
Courtesy of VOLKSWAGEN GROUP OF AMERICA, INC.
Check-valve must remain closed for opposite direction.