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

A All of the above are correct.

You can test whether your vehicle's spring brakes will come on automatically by following the same method you use for testing the low air pressure warning signal. On both tractor-trailer vehicles and single vehicles, step on and off your brake pedal, until you reach an even lower psi reading and the parking brake valve closes or pops out.