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To test the tractor protection valve, charge the trailer air brake system, turn off the engine, and_____

A step on and off the brake pedal several times.
Explaination
To test the tractor protection valve, charge the trailer air brake system, turn off the engine, and lower the air pressure by stepping on and off the brake pedal several times. The tractor protection valve control (also called the trailer air supply control) should pop out (or go from the “normal” to the “emergency” position) when the air pressure falls into the range specified by the manufacturer (typically 20–45 psi).
B flash your high-beam headlights on and off several times.
C keep pressing the brake pedal firmly.