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When you test the tractor protection valve, the red air supply control knob should go into the ________ position.

A emergency

Shut off the engine and step on and off the brake pedal several times to lessen the air pressure in the tanks to test the tractor protection valve. The trailer air supply control knob should pop out, or the valve control should go from the "normal" to the "emergency" position, once the air pressure has dropped enough (usually 20–45 psi).