What happens if one of the service brake circuits of a dual-circuit compressed air-braking system leaks when the vehicle is in motion?

Your selected combination is correct because both points you chose accurately describe the situation when one of the service brake circuits in a dual-circuit compressed air-braking system experiences a leak while the vehicle is in motion.

1. The air supply escaping from the leaking circuit can indeed lead to a significant reduction in braking efficiency, which is critical for vehicle safety.
2. The function of the four-way protection valve is essential. It acts to maintain the integrity of the remaining intact circuits by preventing the pressure from dropping below safety levels, ensuring that the vehicle can still brake effectively even if one circuit is compromised.

Understanding this is vital as it aligns with German traffic regulations, emphasizing the importance of having reliable braking systems in vehicles to ensure the safety of both the driver and others on the road.