What is the minimum requirement for the brake system to be charged at the rear before an air test can start?

The correct answer is 75 PSI because this is the minimum pressure required in the brake system at the rear of the train for air testing to commence. This pressure ensures that there is sufficient air in the system to effectively evaluate the performance and functionality of the brakes. During an air test, it is crucial to have an adequate pressure level to check for any leaks or failures in the system. If the pressure is too low, it may not accurately reflect the operational status of the brakes, leading to potentially unsafe conditions.

In most air brake systems, 75 PSI serves as a baseline that indicates the system is capable of functioning properly during the test. Lower pressures might not provide reliable results, while much higher pressures exceed normal operating conditions, making 75 PSI the appropriate minimum standard for safety and operational integrity.