Figure 7-12.—Vehicle trailer hookup with glad hand cutoff cock.Figure 7-13.—Fifth wheel assembly.trucks, and trailers (fig. 7-12). These valves permit thetractor. They provide the air pressure used to operate theopening and closing of the airlines when towing trailers.trailer brakes by sending air pressure from the air tanksto the brakes via relay valves. The pressure in the serviceTrailer Air Tanksline signals how much pressure the relay valves shouldsend to the trailer brakes. The pressure in the service lineTrailers and dollies have one or more air tanks. Theis controlled by the brake pedal and/or the trailer handair tanks are filled by the emergency air line from thebrake.7-12
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