Which component closes to activate the spring brake when air pressure decreases?

The spring brake is an essential component of a vehicle's air brake system, designed to engage when air pressure drops below a certain level. The parking brake valve is specifically responsible for activating this spring brake mechanism under low pressure conditions.

When the air pressure decreases, the parking brake valve detects this change and responds by closing to ensure that the spring brakes are applied. This is a safety feature designed to prevent the vehicle from rolling when the air pressure is insufficient to hold the regular brakes engaged.

In contrast, the other components mentioned play different roles within the braking system. The brake light switch activates the vehicle's brake lights when the brakes are applied, the traction valves are involved in optimizing traction and stability during braking, and the governor valve regulates the air pressure in the system but does not directly activate the spring brakes. Therefore, the parking brake valve is the correct choice for this function.