What can happen if a truck and trailer are continuously braked with the service brakes on a long downhill slope?

You answered correctly. When a truck and trailer are continuously braked with the service brakes on a long downhill slope, the wheel brakes can overheat. This excessive heat drastically reduces their effectiveness, a situation known as brake fading, which can become very dangerous, especially on long descents. To prevent this, it’s important to use engine braking or auxiliary braking systems (like a retarder or exhaust brake) to help control speed, and to avoid holding the service brakes down for extended periods. This keeps the brakes cool and ensures they remain effective when you need them most.