What condition must be met for spring brakes to engage?

For spring brakes to engage, air pressure must be released from the brake system. In air brake systems, spring brakes are a safety feature designed to automatically apply the brakes if the air pressure drops to a certain level. This typically occurs in the event of a compressor failure, a leak, or other situations that might lead to a loss of air pressure in the system.

When air pressure is released, the powerful spring mechanism within the brake assembly activates and applies the brakes, bringing the vehicle to a stop. This system ensures that a vehicle cannot roll away unintentionally, even when the driver is not present.

The other conditions listed—such as service brake adjustment, brake fluid refill, and temperature thresholds—do not directly affect the engagement of spring brakes. Service brake adjustments relate to the proper functioning of the primary braking system, while brake fluid levels and temperature thresholds pertain more to hydraulic brakes, which operate differently than air brake systems.