Briggs Stratton 9 hp on rear engine snapper. Runs for a couple of minutes then oil/gas mixture blows out of crankcase breather tube... with a lot of pressure. I've seen this problem posted several times and often the o-ring on the dipstick tube is mentioned as the culprit. Why?

If its sucking air in , it has to go out some where. And that's the path of least resistance.