The trailer service brakes will apply whenever the brake pedal is pressed. Often, a hand valve is also fitted on the steering column or the instrument panel of the towing vehicle so the driver can apply the trailer service brakes independent of the towing vehicle’s brakes. When the hand valve and brake pedal are used at the same time, the trailer will receive the higher pressure. Using the brake pedal or hand valve sends air pressure through the trailer service line.
Trailer hand valves must never be used for parking or emergency stops.