What safety feature automatically activates when air pressure drops too low?

The safety feature that automatically activates when air pressure drops too low is the spring brake. In air brake systems, spring brakes are designed as a fail-safe mechanism to secure the vehicle when air pressure decreases below a necessary threshold. These brakes use a set of powerful springs to apply braking force if the air pressure falls, ensuring that the vehicle stops safely and will not roll away.

When air pressure is adequate, the springs are held in a released position by air, allowing normal vehicle operation. However, if there is a failure or leak in the air brake system that causes the pressure to drop, the spring brake engages automatically. This is a crucial safety feature as it helps prevent accidents that could occur due to a loss of braking power, particularly for large vehicles that could become uncontrollable if the air brakes fail.

Other options, such as hydraulic brakes and emergency brakes, do not automatically engage based on air pressure changes. Hydraulic brakes operate through fluid pressure and won't activate due to air pressure drops, and while emergency brakes provide an additional safety measure, they are typically manually operated rather than automatically triggered by air pressure levels. Anti-lock brakes also do not serve this purpose; instead, they help maintain steering control during hard braking, preventing wheel lock-up.