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The Volvo system is unlike most positive systems, as it is not a closed type found on other cars. It does not run any significant negative pressure. The vapours passing through the liquid separator (breather canister on block) and the flame trap (non-turbo) or tee (turbo) are vented directly into the draft of the intake stream. To encourage flow of some blowby gasses directly into the manifold at idle, a small port on the flametrap/tee is connected to a manifold vacuum source. Intake draft will create a vacuum of about 1" H2O. That's less than appears at the lower side of venturi of a standard carburetor.

Gas volume has to increase beyond the flow capacity of open system before appreciable crankcase pressure develops. As deposits build up on the parts, especially the mesh flame arrester used on non-turbo engines, flow is reduced and less blowby is needed to create pressure (and leaks).

What is often perceived to be "wide open" passages would not pass the same tests if checked for ability to pass larger volumes of air/gasses and compared to new parts.