on really hot days, my brakes act really strange. especially in stop and go freeway traffic, and even more so when i have the AC on in these condition... when i push on the brake sometimes, it will go almost all the way to the floor before slowing the car down, if i pick my foot up, then push it again, it will usually engage at a higher point... is this a dying master cylinder? why does it only happen when it's hot? and what does it have to do with the AC?