I agree with Peter that the brakes are not being applied due to the CC function. The CC adjusts the throttle to try to maintain one speed. Going down hill it will let the throttle close back to an idle level if that's what it needs to be sure the engine isn't increasing the speed.
I too would suggest, apply the throttle slightly when you know you are going to be going down a long hill, to allow the vehicle to save kinetic energy, to go a little faster, say 5mph by the bottom. Then slowly let off the throttle as it goes up the next hill.
The best way to see how that works, is to watch the constant MPG readout of the trip display. If you watch that, you will notice that the MPG doesn't drop much when you help it go gain speed going downhill, and likewise, it won't drop that much if you help it to maintain speed going uphill(I mean you control the drop in speed going uphill, to let it fall back to the CC setting before it reaches the top).
The ideal is to have the speed slowly increase going downhill, and slowly decrease going uphill. A CC could be programmed to do that, but it would have to have input about the length of the hills, up and down. That would require some advanced GPS and topography data for all or most roads. I think that's beyond our available data being used for GPS etc.
