you have to take the cup seal out and cut the nipple down, cut it so that the mouth piece when you put it in just goes over the nipple. this will stop most of what you are seeing. I had this same problem. i just had to cut down my new one this morning. otherwise it creates a huge pressure problem and will do exactly what you are describing, i had the same problems. it still happens even after cutting the nipple down but its much better.
what i do is i fill to the top, i put the mouth piece into the tube at the very top and then I suck hard
through the mouth piece while i push it down all the way, that seems to help a bit, still some will end up coming out the bottom probably, I take a napkin or whatever and blow through the mouth piece to force whatever is in the tube to come out the battery connection bottom, wipe it off and good to go.