You really need to learn part 5: Air brake system before reading this. The braking system has parts to control the trailer brakes, in addition to the parts described in Section 5 in combination vehicles. These components are described below.
6.2.1 вЂ“ Trailer Hand Valve
The trailer hand valve (also referred to as the trolley valve or Johnson club) works the trailer brake system. The trailer hand valve ought to be utilized simply to test the trailer brake system. Don’t use it in driving due to the risk of making the trailer skid. The base braking system delivers atmosphere to all or any of this brakes regarding the automobile (such as the trailer[s]). There was significantly less risk of causing a jackknife or skid when working with simply the base braking system.