[mikep]

The way Most "open" PCV systems work is that under high vacuum conditions such as cruise or idle the PCV blocks unburned crankcase gases from cycling into the intake charge. They instead go through a breather tube into the air cleaner assy and are introduced into the intake charge through the carburetor inlet. That is so the loss of vacuum at idle does not kill the engine or casue part throttle detonation. Under acceleration and lower vacuum conditions the PCV valve opens and allows unburned gasses to cycle through the PCV line into the manifold . The breather tube then becomes the air inlet into the crancase and the engine air filter acts as a crankcase air filter to prevent contaminated air from cycling through the motor. This whole system is set up so that crankcase gasses do not escape into the atmosphere but are instead burned. Its a simple emissions control that doesnt harm performance unless you are on the ragged edge of detonation and then PCV should be avoided as misted oil can be present in crankcase gasses and those can agravate detonation. There are other types of PCV systems that are closed and timed systems that either do not run through a breather tube and keep the crankcase in a vacuum or use engine temperature as a basis of when the PCV system is brought online. Dont worry about them. The standard open PCV system is what you are talking about and you need 3 opening. One for a breather to let in filtered fresh air, one for a PCV valve to the manifold and one to pour oil into . The one to pour oil into can be deleted if you have a slip in breather that you can pull out to add oil into the crankcase. Thats what I do on my chevrolet script chrome made in Taiwan valve cover.