A PCV system is what's known as a "closed loop". There is no way for crankcase vapors to be vented to the atmosphere, or for unfiltered air to enter the crankcase. It was initially designed to prevent dust and other contaminants from being drawn into the engine through a vented valve cover and the crankcase ventilation also worked at idle on commercial vehicles such as delivery trucks that seldom if ever reached speeds sufficient to make the road draft tube work effectively (20-30 MPH). After the Clean Air Act of 1968 became federal law, all newly-manufactured vehicles were required to have closed loop ventilating systems to prevent crankcase vapors from being vented into the open air. California went several steps further with their regulations, and required all vehicles operated on public roads to be retro-fitted to a closed loop PCV system. When I ran a shop in central California in the 1970's, I converted hundreds of older vehicles to PCV with a state-approved parts kit so they could pass a roadside smog equipment and exhaust emission inspection.
Jerry
