Turning a Home-brewed MK upside down will disturb all of the yeast from the bottom; so don't do it! The vent at the top is to let air into the keg when the natural carbonation has finished. I used only 3g of Brewing Sugar per litre of brew in the MK and (for me) that was perfect. I gave up using MK's though because they were too small and only lasted at the most four days! The Good News was that it didn't matter if I let air into the MK!