The connections that go from the valve covers directly to the intake tubes are actually the fresh air inlet for the PCV system (they suck filtered air from the intake tubes into the valve covers). I'm not sure if the plastic bits incorporate any kind of one-way valve in them or not though. You could replace those with filtered breathers I guess.
The other pair of hoses involved run from the PCV valves to small ports on the intake manifold, and that's what draws out nasty crankcase vapors via vacuum (and where you'd traditionally install a catch can).
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