Just a comment:
Valve cover breathers will relieve crankcase pressure into the atmosphere, as mentioned, but will but remove corrosive crankcase fumes from the oil pan. The corrosive crankcase fumes are what destroyed the 1930s and 1940s engines well before their time.
Most people only think about not having the oil blow out of the breathers. They forget totally about the corrosive gases.
The old Road Draft Tube method of removing the corrosive gases did a fair job when the vehicle was driven at highway speeds and the Road Draft Tube was designed properly for the vehicle application.
The PCV valve system does a FAR BETTER JOB on that task, is cheap, and just requires hooking up one hose to the intake plenum, then to the pcv valve, and finally to one valve cover. As long as the other valve cover has a breather (for fresh air) on it you are good to go.
There is a TON of history on why you do this stuff for reasons other than emissions.
Just my comment on this subject.
Tom Vaught
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"Engineers do stuff for reasons" Tom Vaught
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