Entwood, thanks for taking the time to explain that so well. and I see what ur saying. But don't see how the pressure in the crankcase is related to the cam covers other than through oil pressure? and not vacuum.
Again I'm keeping an open mind.
Thanks
The oil pressure in the cam covers is zero .. oil pressure ONLY exists in oil ways, drillings, pies etc .. once the oil is sprayed onto a surface, like a cam lobe, the pressure is zero.
Within the cavity that is the cam covers/crankcase there is only air pressure, this increases with the effect of gas blowby from the compression/combustion pressures leaking past the piston rings, and is decreased by the effect of the breather suction we are discussing. The actual "atmosphere" within the crankcase/cam covers cavity comprises a mix of blowby gases/oil mist/water vapour.
If the pressure builds up due to blocked breathers, we know the cam cover gaskets are the weakest link, so they fail and leak the "atmosphere" from within to without .... the blowby gases will simply escape, the water vapour will evaporate fully, the oil mist will condense and stick to any surface it can, then run and appear as a leak. If the cam cover gasket hole is large enough some of the lubricating oil spray from within may well leak out directly through that hole.
The answer is to maintain the slight negative pressure that the "vacuum" from the plenum causes via the breather system, this will "suck" the "atmosphere" out along the breather pipes until it gets burned back in the cylinders ... unfortunately for us .. the temperature change from a hot crankcase to a cold(ish) plenum cause the oilmist to condense as the horrible emulsified mess that we get, that if not cleaned out blocks the airways we want to do the job !!