At what PSI is an air brake system considered fully charged?

Answer: C — 125 PSI

An air brake system is considered fully charged at 125 PSI because this is the standard operating pressure for most commercial vehicle air brake systems. When the system reaches this pressure, it ensures that there is sufficient air supply for the brakes to function effectively. A fully charged system at this pressure allows for optimal brake performance and readiness for use. Operating below this pressure can lead to insufficient braking force and may impair the vehicle's ability to stop safely. Therefore, maintaining the air brake system at or above 125 PSI is crucial for safety and compliance with regulations governing commercial vehicles.