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Question:

What is the best way to test if your vehicle's spring brakes come on automatically?

A All of these are correct.
Explaination

On both tractor-trailers and single vehicles, to test whether the spring brakes will come on automatically, use the same method you use for testing the low air pressure warning signal: Step on and off the brake pedal, this time until you reach an even lower psi reading and the parking brake valve closes or pops out.