During a pre-trip inspection of a vehicle with air brakes, you fan the brakes to reduce air pressure. At approximately what pressure should the spring (parking) brakes automatically activate?

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TheoryPractice · Accepted Answer

40 psi. The CDL manual states that spring brakes should automatically activate (pop out) when air pressure falls into the range of approximately 20 to 45 psi, with the generally tested threshold around 40 psi, ensuring the vehicle is held if air is lost.

During a pre-trip inspection of a vehicle with air brakes, you fan the brakes to reduce air pressure. At approximately what pressure should the spring (parking) brakes automatically activate?

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