During a dual air brake system pre-trip inspection, how do you verify the tractor protection valve operates automatically?

1. By driving at 30 km/h and stepping on the brake pedal
2. By shutting off the engine, releasing the parking brakes, and pumping the service brake pedal repeatedly until air pressure drops to between 20 and 45 psi, at which point the trailer supply valve (red knob) must pop out
3. By disconnecting the electrical cable while the engine is running
4. By checking if the valve is free of rust