What should be the allowable leakage rate at the bottom of the pressure protection valve?

The allowable leakage rate at the bottom of the pressure protection valve should be zero, meaning there should be no leakage. This is crucial because the pressure protection valve plays a vital role in maintaining the proper air pressure in the braking system. Any leakage could compromise the effectiveness of the air brakes by reducing the available air pressure needed for safe operation. A tight seal ensures that the system maintains pressure, which is essential for the reliable operation of air-operated components in the braking system. Thus, ensuring there is no leakage at the pressure protection valve directly supports the safety and functionality of the bus's braking system.