To check if the spring brakes come on automatically, what should you do?

Pumping the brake pedal to reduce air pressure in the tank is the correct method to check if the spring brakes come on automatically. This action lowers the air pressure in the braking system. Most heavy-duty vehicles equipped with air brakes have a safety feature in which the spring brakes are designed to engage when the air pressure drops to a certain level. By reducing the air pressure through pumping the brake pedal, you can observe if and when the spring brakes will activate as a safety precaution. This operational check is crucial for ensuring that the brakes will function properly in case of an air pressure loss while driving.

The other choices do not effectively assess the automatic engagement of the spring brakes. Increasing air pressure in the tank would actually keep the brakes disengaged, and checking fluid levels pertains more to hydraulic systems, not air brakes. Testing the brake lights is unrelated to checking the functionality of the braking system itself.