The PCV valve is nothing more than a check valve. On stock application the turbo pressure keeps the valve open. When re-routed the check valve will open when needed and release the pressure. Gassers need the vent due to high rpm's. Diesels however don't turn enough rpm's to justify catch cans and an outside source to ventilate the system. In other words the PCV valve is not necessary. Air travels one way through the valve. Unless you are driving through water just hook up hoses to the ports without the valve and let it breathe. PCV was created by the EPA and we all know how well they work.
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PCV (positive crankcase ventilation) and the PCV valve has been around much longer than EPA. Since the late 1920's motor vehicles engines have had some type of PCV system. Usaully it was simply a breather cap and a draft tube. The draft tube was positioned in such a way that air moving across the opening from either the fan or the vehicles travel would draw a vacuum clearing vapor from the crankcase.
Closed PCV with developed during WWII so that tanks could cross deeper water bodies. Closed PCV system were not used for emission until 1961 in California (of course), the rest of the states 1962. EPA was formed in 1970 under Nixon.
A diesel motor doesn't normally need a PCV valve. A PCV valve purpose is not on-off, but to reverse the manifold vacuum signal. An internal restrictor (generally a cone or ball) is held in "normal" (engine off, zero vacuum) position with a light spring, exposing the full size of the PCV opening to the intake manifold. With the engine running, the tapered end of the cone is drawn towards the opening in the PCV valve by manifold vacuum, restricting the opening proportionate to the level of engine vacuum vs. spring tension. At idle, the intake manifold vacuum is near maximum. It is at this time the least amount of blow by is actually occurring, so the PCV valve provides the largest amount of (but not complete) restriction. As engine load increases, vacuum on the valve decreases proportionally and blow by increases proportionally. With a lower level of vacuum, the spring returns the cone to the "open" position to allow more air flow. At full throttle, vacuum is much reduced, down to between 1.5 and 3" Hg. At this point the PCV valve is nearly useless, and most combustion gases escape via the "breather tube" where they are then drawn in to the engine's intake manifold anyway
The Duramax doesn't need a PCV valve. The vacuum for the PCV system is the differential between the turbo and the air filter. In this case the vacuum level follows throttle level. The more throttle, the higher the differential, the higher the vacuum. A PCV valve to reverse the vacuum signal is not need in this system. I have not taken the housing on valve cover apart but I believe all that you will find inside is baffling and maybe a check valve. A check valve and PCV valve are not the same thing.
Not circulating air through the block leaves the blow-by gases in the crankcase to commentate the oil.