I speak as one who has done the research but claims no expertise. Here's how I understand the system. At idle the engine produces relatively high vacuum that opens the PCV valve to suck out crankcase vapors. The filtered vent on one valve cover allows outside air to replace that being sucked out. At high RPM crankcase pressure increases and the PCV valve allows that pressure to be released into the intake for the vapors to be burned. If the pressure exceeds the flow capacity of the PCV valve, excess pressure is vented through the filtered valve cover vent. . . . So there are times when air in entering the engine through the valve cover vent, and times when vapor is exiting. ... Lastly, the PCV valve is designed to allow flow only one way to prevent a backfire in the intake from traveling back to the combustible vapors in the crankcase.
The Cobra I used to have was built with a racing setup that included catch cans for fluids. The engine vent system consisted of baffled fittings on each valve cover that were plumbed to a remote catch can with a filter vented top. It served the purpose of allowing the crankcase to vent at high RPM, but did nothing to provide clean air at lower speeds. It would accumulate 2-3 ounces of fluid after a weekend of track time. That system seemed right for a race car that would have the
oil changed frequently, but not a street car.