Quantcast Trailer Air Lines

Trailer Air Lines Every combination vehicle has two air lines. These lines are the service line and the emergency line. They run between each vehicle, such as tractor to trailer, trailer to dolly, and dolly to second trailer. SERVICE AIR LINES.— The service line carries air that is controlled by the foot brake or the trailer hand brake.  Depending  on  how  hard  the  foot  brake  is engaged, the pressure in the service line will similarly change.  The  service  line  is  connected  to  relay  valve(s) on the trailer to apply more or less pressure to the trailer brakes. As pressure increases in the service line, the relay valve opens and sends air pressure from the trailer air tanks to the trailer brake chambers, thus applying the trailer  brakes. EMERGENCY  AIR  LINES.—  The  emergency line has two purposes. First, it supplies air to the trailer air  tanks.  Second,  the  emergency  line  controls  the emergency  brakes  on  the  combination  vehicle.  Loss  of air pressure in the emergency line causes the trailer emergency  brakes  to  activate.  The  pressure  loss  could be caused by a trailer breaking loose and tearing apart the emergency air hose. The loss could also be the result of a hose, metal tubing, or other parts breaking and causing an air leak. When the emergency line loses pressure,  it  also  causes  the  tractor  protection  valve  to close, causing the air supply knob to pop out. Emergency lines such as hoses couplers, and other parts, have a red covering. The red covering allows you to separate the emergency lines from the service lines which have a blue covering. Hose  Couplers Hose couplers, commonly known as glad hands (fig. 7-11), are coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer. The glad hands have rubber seals, known as rubber grommets, that prevent the air from escaping. Clean the rubber grommets before you connect the glad hands. When connecting the glad hands, press the two seals together with the glad hands at a 90-degree angle to each other. A turn of the glad hands attached to the hose joins and locks the couplers. Some  vehicles  have  “dead  end”  or  dummy  glad hands to which the hoses should be connected when not in use. This prevents water and dirt from getting into the glad hands and the air lines. This is very important because keeping the air system clean is a critical factor. Figure 7-11.—Glad hands. When connecting the glad hands, ensure the proper glad hands are coupled together. On some equipment, metal tags are attached to the lines with the words service and emergency stamped on them. The color blue is used for the  service line and the color red for the emergency line  connections. If the air lines are crossed, supply air is sent to the service line instead of going to charge the trailer air tanks; therefore, air is not available to release the trailer spring brakes (parking brakes). If the spring brakes do not release when you push the trailer air supply control knob, check the air line connections. CAUTION Older trailers do not have spring brakes. If the air supply in the trailer air tanks has leaked out,  emergency  brakes  will  not  exist,  and  the trailer wheels will turn freely. If you cross the air lines, the trailer will roll; however, there will be no trailer brakes. NOTE: Always test the trailer brakes before driving by engaging the hand valve or by pulling the tractor protection valve. Once these brakes are engaged, shift the tractor to low gear and pull gently against the brake system to make sure the brakes work. Shutoff Valves Shutoff valves, commonly known as cutoff cocks, are used in the service and emergency lines glad hands located on the back of military series tractors, cargo 7-11


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