Why is a decrease of pressure of more than 0.7 bar problematic in the compressed air system of a motor vehicle, if you press the brake pedal fully down one time (cut-off pressure around 10 to 13 bar)?

You answered correctly. In compressed air brake systems, if the pressure drops by more than 0.7 bar after pressing the brake pedal once, this is a sign of a significant leak or malfunction. This rapid loss means there may not be enough air left for further braking, and the effectiveness of the brakes can decrease, resulting in longer stopping distances or even brake failure. A drop this large does not create a vacuum—the system is designed only to use compressed air. Therefore, the first and second answers are correct because they describe the real risks associated with such a pressure loss: insufficient air for additional braking and worsened braking performance. This checks the safety of the braking system and ensures the vehicle remains controllable.