Trailer Air LinesEvery combination vehicle has two air lines. Theselines are the service line and the emergency line. Theyrun between each vehicle, such as tractor to trailer,trailer to dolly, and dolly to second trailer.SERVICE AIR LINES.— The service line carriesair that is controlled by the foot brake or the trailer handbrake. Depending on how hard the foot brake isengaged, the pressure in the service line will similarlychange. The service line is connected to relay valve(s)on the trailer to apply more or less pressure to the trailerbrakes. As pressure increases in the service line, therelay valve opens and sends air pressure from the trailerair tanks to the trailer brake chambers, thus applying thetrailer brakes.EMERGENCY AIR LINES.— The emergencyline has two purposes. First, it supplies air to the trailerair tanks. Second, the emergency line controls theemergency brakes on the combination vehicle. Loss ofair pressure in the emergency line causes the traileremergency brakes to activate. The pressure loss couldbe caused by a trailer breaking loose and tearing apartthe emergency air hose. The loss could also be the resultof a hose, metal tubing, or other parts breaking andcausing an air leak. When the emergency line losespressure, it also causes the tractor protection valve toclose, causing the air supply knob to pop out.Emergency lines such as hoses couplers, and otherparts, have a red covering. The red covering allows youto separate the emergency lines from the service lineswhich have a blue covering.Hose CouplersHose couplers, commonly known as glad hands(fig. 7-11), are coupling devices used to connect theservice and emergency air lines from the truck or tractorto the trailer. The glad hands have rubber seals, knownas rubber grommets, that prevent the air from escaping.Clean the rubber grommets before you connect the gladhands. When connecting the glad hands, press the twoseals together with the glad hands at a 90-degree angleto each other. A turn of the glad hands attached to thehose joins and locks the couplers.Some vehicles have “dead end” or dummy gladhands to which the hoses should be connected when notin use. This prevents water and dirt from getting into theglad hands and the air lines. This is very importantbecause keeping the air system clean is a critical factor.Figure 7-11.—Glad hands.When connecting the glad hands, ensure the properglad hands are coupled together. On some equipment,metal tags are attached to the lines with the wordsservice and emergency stamped on them. The colorblue is used for the service line and the color red for theemergency line connections.If the air lines are crossed, supply air is sent to theservice line instead of going to charge the trailer airtanks; therefore, air is not available to release the trailerspring brakes (parking brakes). If the spring brakes donot release when you push the trailer air supply controlknob, check the air line connections.CAUTIONOlder trailers do not have spring brakes. Ifthe air supply in the trailer air tanks has leakedout, emergency brakes will not exist, and thetrailer wheels will turn freely. If you cross theair lines, the trailer will roll; however, there willbe no trailer brakes.NOTE: Always test the trailer brakes before drivingby engaging the hand valve or by pulling the tractorprotection valve. Once these brakes are engaged, shiftthe tractor to low gear and pull gently against the brakesystem to make sure the brakes work.Shutoff ValvesShutoff valves, commonly known as cutoff cocks,are used in the service and emergency lines glad handslocated on the back of military series tractors, cargo7-11
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