What safety feature is used to prevent the application of brakes if air pressure drops too low?

Answer: B — Low air pressure warning system and spring brakes

The low air pressure warning system and spring brakes serve as a crucial safety feature in brake systems, particularly on vehicles such as school buses that rely on compressed air for operation. This system activates a warning light and/or an audible alarm to alert the driver when air pressure in the system falls below a safe threshold. If the air pressure continues to drop, the spring brakes activate. These brakes are engaged by powerful springs, which automatically apply the brakes in the event of a loss of air pressure, preventing the vehicle from moving uncontrollably. This safety design is particularly important in maintaining control of the vehicle and preventing accidents, as it operates independently from the normal brake application mechanism, ensuring that the bus can come to a stop even in the case of a significant air pressure failure. Having such features in place mitigates the risk of brake failure due to inadequate air supply, providing both a warning and a fail-safe mechanism that enhances overall vehicle safety.