Hello,
Draekon got it right .. If you remove the air cleaner cover ( where the filter is ) and look into the hole which the air flows into the breather .. you'll see the trap door closed if the engine is off. The trap door will be open if the engine is running, if the vacuum motor is working.
Does anyone know the reason why this was designed to close so completely ?? Then open right up when the engine starts ?? Maybe to keep the mice out if it sets for a long time ..
IRON HORSE
It was designed for pollution control. As soon as the engine is turned off, all the remaining gasses and vapors are trapped in the air cleaned and are supposed to be collected by the charcoal canister. This is how some of the last of the carbureted cars in the late 80s still passed EPA standards before fuel injection was required.
There are actually two vacuum operated doors on the air cleaner. The one rmick mentioned is right above the stand pipe, and won't completely block off air flow. The pollution control one is closer to the center and will close all the way when there is no vacuum.
That still doesn't explain the oil in the air cleaner though. I suspect blow-by. My engine starting getting super oily in the air cleaner that it would completely saturate the felt wad of the breather everyday. I rebuilt my engine and haven't had any trace of oil on the breather since.