Managing Tractor and Trailer Air

Cab Controls for Trailer Braking

Operating combination vehicles requires precise control over air systems shared between the tractor and the trailer. In the cab, you will find controls specifically dedicated to managing this connection. The trailer hand valve, also known as the trolley valve or Johnson bar, operates only the trailer brakes. Because it does not apply the tractor brakes, it must never be used while driving down the road. Using the hand valve while driving can cause the trailer to skid. Instead, use your normal foot brake during regular driving. The foot brake sends air to all brakes across the entire combination, which creates much less danger of a skid or jackknife.

The trailer hand valve has one primary purpose: testing the trailer brakes before you begin your trip. It must never be used as a parking brake. Air can slowly seep out of the system, and on trailers that do not have spring brakes, that gradual air leak will release the brakes completely and allow the vehicle to roll away. Drivers must always set the actual parking brakes when parking a combination vehicle. If you are parking an older trailer that lacks spring brakes, you must also use wheel chocks to keep the vehicle securely in place.

Before pulling out onto the road, you need to verify that the trailer brakes are actually holding. You can test them using the hand valve, or by pulling out the trailer air supply control to set the trailer emergency brakes. With the trailer brakes set, put the tractor in a low gear and pull gently against them. Pull gently against the set trailer brakes in a low gear to check that they work; this does not replace other brake checks.