Originally Posted by Mountainboy

Vacuum drops under high engine load, like going up a large hill, driving at high altitude, and rapid acceleration. Think of the tank as storage......When the engine is not producing much vacuum under load, the vacuum stored in tank still allows the devices that use manifold vacuum to operate. Emission control devices that run off of or are affected by manifold vacuum include EGR valve, carbon cannister, and PCV valve. A good insurance policy against having any issues with your emission control system is to make sure that there are no leaks anywhere. If it were my car, I would replace it.