What is the purpose of the trailer hand valve (Johnson Bar)?

The purpose of the trailer hand valve, commonly known as the Johnson Bar, is to test the trailer brakes. This device allows the operator to manually apply the trailer brakes independently of the tow vehicle's braking system. By using the hand valve, the operator can check whether the trailer brakes are functioning correctly before beginning to tow, which is essential for safety. This function is crucial for ensuring that the brakes are capable of stopping the trailer effectively, especially in emergency situations or when traveling downhill.

The ability to test the brakes independently helps in identifying issues with the trailer braking system that could pose safety risks during operation. Thus, having this functionality built into the braking system contributes significantly to the overall safety of the towing operation.