I don't slam my brakes often so I've only noticed this a few times at low speeds (i.e. backing out of a parking space) and can't reproduce it with any consistency. Sometimes when pressing the brake pedal all the way down it will make a hissing sound and a very limited amount of pressure will be applied to the brake pads (as evidenced by the vehicle only gradually stopping even after the pedal has been fully depressed rather than stopping immediately, when braking started while moving less than 5mph).