This air, plus the "blow by" from the cylinders, is pulled into the air cleaner and on to the carburetor to be re-burned by way of the breather on top of the oil filler. Draw of air into the upper chamber of both the oil-filled air cleaner and the later paper types creates a slight vacuum inside the air cleaner. This vacuum was enhanced in 1973 and later models by a vacuum line running to the air cleaner from a port in the intake manifold right below the carburetor.
This negative pressure in the air cleaner, plus the positive pressure from the spiral groove pulling air into the crankcase, results in the oil staying put in the crankcase. Fumes are burned as they are sucked into the carburetor by way of the breather and the air filter.
With little or no pressure drop at the top of the breather because of substitution of the stock air cleaner with an aftermarket one, any extra pressure in the crankcase causes the oil mist to push out past the spiral groove and through the slots behind actually, in front of the pulley. This oil mist, of course, settles out inside the engine compartment and can make a real mess.
So, for the crankcase ventilation system to work properly, it is essential that 1 a vacuum is established in the air cleaner, and 2 the hose between the oil filler and the air cleaner be in place so that crankcase gases are sucked up into the air cleaner and thence into the carburetor and intake manifold for burning.