Emission Control Systems: Description and Operation

Fig. 39 Vacuum vent valves:

Vacuum vent valves, Fig. 39, used on some vehicles, control the induction of fresh air into a vacuum system to prevent chemical decay of vacuum diaphragms that can occur on contact with fuel vapors. Two types of valves are used, the black color coded valve is a vent valve only, while the natural color coded valve is a combination vent and delay valve.
These valves should be installed with ports pointing downward. The vacuum source should be connected to the cap port, and the system or device operated by the valve to the body port.