General question for the 727 gurus. If one was to put very low pressure air (like a hose and your lungs) into the dipstick tube would that prove the vent is open if it did not pressurize?
I don't know for sure about that, but if so, maybe you could rig up a cork plug with a tire valve and use a bicycle tire pump with a gauge to check it.
I wonder if the front pump gasket is idiot proof, or if one could inadvertently put the gasket in wrong and cover up the vent passage.