I have a 2014 base and after years of this annoying me I decided to try to break the car (while still under warranty). I spend a week doing bad things. Nothing different happened...... Untill..... I was stopped at a light. I did the double pump thing so the pedal was real high. Then held full max pressure on the pedal. After a few seconds the pedal dropped away. All the way to the floor. It felt like a blew a line. I didnt blow anything. The reservoir was lower, but not empty or even low enough to think that was an issue. But the pedal has been normal since. (almost a year). It has never done that fade to the floor thing again. I have tried. It never leaked. I am GUESSING that one of the pistons had been stuck since the car was new. Since your car is under warranty..... Maybe you should try pushing the pedal to the floor. I mean it, all the way down.