What should be the minimum air pressure before operating a vehicle equipped with air brakes?

Answer: B — 90 psi

The minimum air pressure before operating a vehicle equipped with air brakes should be at least 90 psi. This pressure is crucial for safe brake operation and ensures that the air brake system can function properly. At levels below this threshold, there is a significant risk that the brakes might not respond effectively, which compromises safety. The air brake system operates by using compressed air to engage the brakes, and inadequate pressure can lead to ineffective braking power or even failure to brake. Maintaining a minimum air pressure of 90 psi helps ensure the system is fully operational and can handle the requirements of driving safely, especially under various conditions or loads.