Question What is the Johnson bar? Answers A hand valve that works the trailer brakes A valve that indicates the fuel level A valve that limits the amount of cargo Explanation The trailer hand valve, also called the trolley valve or Johnson bar, is used to work the trailer brakes. It should be used only to test the brakes. Using it while a vehicle is in motion could cause a skid. Take a FREE Practice Test Now This questions appears in the following tests: Rhode Island Double , Oklahoma Combination , South Carolina Combination , Utah Combination ,