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Question:

How should you check that the spring brakes come on when the air pressure in the system drops below a certain level?

A Park on level ground, chock the wheels, release the parking brake when you have enough air pressure, shut the engine off, and repeatedly press and release the brake pedal.
Explaination

When doing the final check of your air brake, you should cause your vehicle to have low air pressure, to be sure the warning signal comes on, by turning the engine off, with the wheels chocked, and pushing on and off the brakes to release the air pressure. Once you're sure the low-pressure warning is viable, continue doing the same until the spring brakes come on automatically, usually between 20- 45 psi.