I have CCB's on my F80 and the only time I experience that lack of braking is right after washing it. I haven't had that experience in the rain or not nearly as bad. There is a BMW video explaining the sounds and other peculiarities of CCB's including this as well as a whale sound in slow driving after a car wash. Search "How to use your high performance brake system." I definitely experience that as well. They say you just need to stand on it and that the braking power is the same but takes more force. I brake repeatedly for a bit and they get right back to normal. Actually happened today and does get the heart rate up. Another reason to be careful about letting anyone drive your car!
