I'm no big expert on this, but I think that in cars equipped with a PCV valve -- as opposed to the later system with a metered orifice at the manifold connection -- the flow is always into the intake manifold. The valve controls this. At low loads (high vacuum), it just barely opens so there is no significant false air into the combustion mixture. At high loads (low vacuum), it opens wide so flow into the intake still overcomes any blowby pressure. There is always some vacuum or the car wouldn't run.

If ever there is positive pressure in the intake in case of a backfire through the carbs, the valve closes to prevent any possibility of lighting off fumes in the crankcase.

It doesn't matter what filters the air returning to the crankcase. If you can hook up to a regular air cleaner, great. If not, a separate mini-filter works fine.