Check Valve, Checking

WARNING: This page is about a different car, the 2021 Audi S4 and 2021 Audi A4 Quattro. However, it is still accessible from the selected car via links, so may be relevant.

The valve must allow air to flow through in the direction of the vacuum pump.
Valve must remain closed in the direction of the brake booster.

NOTE:

Note the flow direction: The arrow symbol faces the vacuum pump.