Which component engages the brakes when air pressure is lost in the spring brake system?

The spring mechanism is a critical component of the spring brake system, designed to ensure safety in the event of a loss of air pressure. When the air pressure in the braking system drops below a certain level, the spring mechanism is activated. This mechanism uses heavy springs that are compressed when air is present to apply the brakes. If air pressure is lost, these springs expand, thus engaging the brakes automatically. This feature is crucial for maintaining control and minimizing the risk of accidents, as it ensures that the vehicle will stop safely even if there is a failure in the air system.

In contrast, brake fluid is a hydraulic fluid used in hydraulic brake systems, not applicable to air brake systems. The anti-lock braking system is focused on preventing wheel lock-up during braking and does not directly relate to engaging brakes due to air pressure loss. Pneumatic power refers to the use of compressed air, but it is not the mechanism that engages the brakes when air pressure fails. Instead, it's the spring mechanism that plays the essential role in this safety feature.