Air Brake Practice Test

When braking on a long steep downgrade, it is essential to make use of engine braking in conjunction with the brakes to maintain control and safety. The correct approach is to utilize the engine's braking effect, which helps slow the vehicle by downshifting to lower gears, thus reducing the reliance on the vehicle’s brake system alone.

By maintaining the speed approximately 5 mph below a predetermined stable speed, the driver can ensure that the brakes are not overheating and can effectively handle sudden increases in speed due to gravity. This strategy prevents brake fade, which can occur from the excessive heat generated by constant braking, particularly on steep descents. This method of combining engine braking with controlled use of the service brakes helps in safely managing the vehicle's speed without putting undue stress on the brake system.

Other choices either promote less effective braking techniques or do not provide the necessary control for heavy vehicles going down a steep grade, thus making option D the most prudent and effective choice for safety in this scenario.