You guys who haven't filed your M tips, is the end of the tip that sits against the cap completely flat on yours?
Mine was, so when I sucked on it, the cap was sucked up flush and formed a seal, like putting a flat piece of metal over the end of a vacuum cleaner if that makes it clearer.
Maybe my cap is looser than it should be, so it pulls up against the tip and seals instead of staying a little out?
Something's got to be different about yours than mine I'm sure, it just makes no mechanical sense tome otherwise, the harder I sucked, the harder the cap was pulled up to the tip and the tighter it was sealed lol.