The vials should be sealed and in vacuum status. you only inject the same amount of air that you intend to draw out of product. you need 4ius, inject 4ius of air before pulling the 4ius of product out. you want to maintain as much of a constant vacuum in the vial as possible at all times to prevent the growth of any bacteria and or microorganisms. it you are bursting your glass vial you have WAY too much air in there. put a dry pin in there and pull out as much air as you can or until the plunger starts to fight with you and that should be a sufficient vacuum.