Positive airflow works either way- - - -the system doesn't really care which way the air gets into the engine. Internally, a PCV valve consists of a spring-loaded tapered plug that takes small gulps of air from the crankcase (or valve cover) as the manifold vacuum pulsates slightly between intake strokes. Otherwise, there would just be a massive vacuum leak and the engine wouldn't idle worth a flip. Getting clean, filtered air into the engine is the trick, either by adding a filter at the inlet, or just pulling air from the "clean" side of the main air cleaner. That's how we did thousands of conversions to PCV in California back in the 1970's when the California Air Resources Board ("CARB"- - - -how's that for irony?) decreed that all engines on California roads had to be retrofitted for sealed crankcase ventilation systems.
Jerry
