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)?

Your answer is correct because in German traffic regulations, maintaining sufficient air pressure in the compressed air brake system is essential for safe braking.

When you press the brake pedal fully once, if the pressure drops by more than 0.7 bar, it means:

1. The air supply is decreasing too quickly. Normally, the system should retain enough pressure so that if you need to brake again soon after, there is sufficient compressed air available for effective braking.

2. The braking effect deteriorates because lower air pressure means less force is available to apply the brakes effectively. This can lead to longer stopping distances or even brake failure.

The third option about vacuum does not apply here because compressed air brake systems operate with pressurized air, not vacuum.

In summary, a pressure drop greater than 0.7 bar signals that the system is losing air too rapidly, compromising the reliability and performance of the brakes, which is why it is considered problematic.