If the service brake of the towing vehicle has at least two independent loops and one loop fails, what happens to the trailer brakes?

1. The trailer brakes are permanently locked on until a workshop resets them
2. The trailer is left unbraked so the combination can coast
3. The loop unaffected by the failure engages the trailer brakes partially or in full
4. Only the tractor's horn sounds; the trailer is unaffected by design