Yes, it'll use one or both, depending upon just how much it has to hold the set speed. You really don't want the brakes doing all the work on a long downgrade. That's a well known fact in the trucking industry, which I was in for 31 years. That's why there are jake brakes (aka engine brakes) on big trucks. Brakes on fire and 80,000 lbs on a downgrade just ain't no fun at all.I've not noticed the car applying the brakes, but have had it downshift the transmission on a long downhill while using the cruise control.