What happens if the service brake is activated while the permanent brake is switched on?

You answered correctly. When the service brake is used while the permanent brake (such as a retarder or engine brake) is still on, both braking systems combine their effects. This results in greater braking force being applied to the drive axle wheels. In German traffic regulations and vehicle design, this is intended to improve braking performance, especially with heavy vehicles. However, you should always be cautious, as applying too much force to the drive axle can risk wheel lock-up, especially in slippery conditions. The permanent brake is not automatically switched off when you use the service brake; instead, their forces add up. This rule helps you understand how vehicle braking systems interact and why it's important to use them correctly on the road.