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Question:

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

A All of the above are correct.
Explaination

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