We do this every year... and we always stay at CBR. We've caught the bus around 7 a.m., and got there early. (The buses might even start earlier than that. I can't remember.)
Last year, we caught the bus at 7 a.m. and were at the park, in line at about 7:15. They didn't start letting breakfast people in until about 7:45. (It was raining that day, so not sure if that time is the norm or not.)
You can always ask one of the resort CMs when the buses start running. Or better yet, after check-in, scoot on over to the Concierge and ask them.