When coupling a tractor to a semitrailer, what is the primary purpose of charging the trailer's air system and then pulling the air supply knob (tractor protection valve)?

1. To verify the electrical connection is providing power to the trailer lights.
2. To confirm the fifth wheel jaws have securely locked around the kingpin.
3. To ensure the trailer's service brakes are not leaking air.
4. To check that the trailer emergency brakes will apply if the trailer breaks away.