I have an 06 supercharged cobalt and I'm having trouble with my brakes as I drive the more I use my brakes the stiffer they get until there rock hard and I can hardly move the car and it only takes about five minutes for it to get that bad. If I let the car sit for about twenty minutes the pressure goes down so far that I can push them straight to the floor but then they lock right back up again. I was told it was the calipers so I replaced them and it is still doing it. Anyone have any ideas?