Out of the blue, the brake goes to the floor

As per previous questions and issues on this problem--the latest one July of 2013--we have had these exact same problems and results. It's on its third MC and have replaced the vacuum booster. It works great for a while, and then out of the blue, the brake goes to the floor. Of course it stops by the time we get it into the dealer. All you have to do is pump the brake once, and they're back up. In a panic stop, you don't always have time to do that. The dealer can't figure it out, and we're tired of leaving our car for days at a time for them to drive all over the place-- and it doesn't go out on them in that short amount of time. KIA needs to wake up and realize this is a big problem before someone gets killed. Could it be the ESC (electronic stability controller)? That's big bucks. We've already put in close to $2000, and the problem keeps returning. Any better ideas?