Vacuum Check Valve: Description
WARNING: This page is about a different car, the 2005 Jeep Liberty. However, it is still accessible from the selected car via links, so may be relevant.
A vacuum check valve is installed in the accessory vacuum supply line in the engine compartment, near the vacuum tap on the engine intake manifold, and at the HVAC housing takeout. The vacuum check valves are designed to allow vacuum to flow in only one direction through the accessory vacuum supply circuits.