2005 Dodge Durango, brakes do not engage when driving?

Very rare, but when driving the vehicle and applying the brakes to stop, the pedal will have no pressure and go all the way to the floor. Immediately, pumping the brakes engages them. It happens very seldom, but the dealer has checked it many times, fluid level is good and the vehicle computer was changed. It still does it though. The pedal doesn't vibrate or offer any resistance at all. Durango limited, anti-lock brakes. Any Ideas?