You have reached your safe speed of 40 mph as you are driving down a steep downgrade. When would you apply the service brake?

If your "safe" speed is 40 mph, you would not apply the brakes until your speed reaches 40 mph. You now apply the brakes hard enough to gradually reduce your speed to 35 mph and then release the brakes. Repeat this as often as necessary until you have reached the end of the downgrade. [California Commercial Driver Handbook]