Final answer:
To maintain control and prevent brake overheating on a steep downgrade, after reaching a safe speed like 40 mph, you'd apply the service brakes until your speed drops to 35 mph before releasing them, allowing your vehicle to coast back to the target speed.
Step-by-step explanation:
When driving down a steep downgrade and having reached a safe speed of 40 mph, to ensure that you maintain control of the vehicle and the speed does not build up, you would use a technique called 'brake feathering' or 'engine braking' where applicable. However, if you are specifically using the service brakes (often referred to simply as 'brakes'), a common safe practice is to reduce your speed by about 5 mph below your target speed before releasing the brakes. So if you are at 40 mph, you would apply the service brakes until your speed drops to 35 mph, then release the brakes to allow the vehicle to coast back up to the target speed (in this case, 40 mph), before repeating the process as necessary. This technique helps to prevent overheating of the brakes during a long descent.