What is the acceptable air pressure range for school bus parking brakes and service brakes?

The acceptable air pressure range for school bus parking brakes and service brakes is indeed 90 to 120 psi. This range is important because it ensures that the braking system operates effectively and safely. When the air pressure falls below 90 psi, there may not be sufficient pressure to engage the brakes fully, potentially leading to a hazardous situation during braking maneuvers.

Additionally, maintaining pressure above 120 psi is equally crucial to prevent issues like excessive wear on brake components or, in extreme cases, brake failure due to insufficient air supply. Therefore, the range of 90 to 120 psi provides a safe and efficient operating standard that allows for responsive braking and reliable parking brake engagement on school buses.

In the context of the other options, while ranges like 80 to 100 psi or 70 to 110 psi may seem reasonable, they do not align with the industry standards set for safe and effective braking performance. The higher ranges like 100 to 130 psi might introduce risks of over-pressurization, which can also lead to mechanical failures or safety concerns. Thus, the chosen range best supports safe operational practices.