Air Brake Practice Test

When air pressure decreases greatly in a vehicle equipped with air brakes, the spring brakes engage, causing the brakes to come on. This is a safety feature designed to activate the brakes in case of an air pressure failure. The spring brakes rely on powerful springs that are held in the released position by air pressure; when that pressure drops significantly, the springs force the brake mechanism to engage, effectively applying the brakes to help prevent the vehicle from moving uncontrollably.

In contrast, the other options reflect scenarios that do not align with the operation of air brake systems. For example, brakes releasing would occur with an increase in air pressure, while brakes locking up or failing would not typically be the intended outcomes of decreased air pressure. Engaging the spring brakes ensures that the vehicle remains safe under critical conditions, thereby allowing the driver to maintain control.