in the cab. The control valve allows you to open  and shut the tractor protection valve. The tractor  protection valve will close automatically if air  pressure is low (in the range of 20 to 45 psi). When  the tractor protection valve closes, it stops any air  from going out of the tractor. It also lets the air out  of the trailer emergency line. This causes the trailer  emergency brakes to come on, with possible loss  of control. (Emergency brakes are covered later.)  Emergency lines are often coded with the color red  (red hose, red couplers, or other parts) to keep  from getting them mixed up with the blue service  line. The trailer air supply control on newer vehicles is a  red eight-sided knob, which you use to control the  tractor protection valve. You push it in to supply the  trailer with air, and pull it out to shut the air off and  put on the trailer emergency brakes. The valve will  pop out (thus closing the tractor protection valve)  when the air pressure drops into the range of 20 to  45 psi. Tractor protection valve controls or  "emergency" valves on older vehicles may not  operate automatically. There may be a lever rather  than a knob. The "normal" position is used for  pulling a trailer. The "emergency" position is used  to shut the air off and put on the trailer emergency  brakes.  Every combination vehicle has two air lines, the  service line and the emergency line. They run  between each vehicle (tractor to trailer, trailer to  dolly, dolly to second trailer, etc.)  Service Air Line. The service line (also called the  control line or signal line) carries air, which is  controlled by the foot brake or the trailer hand  brake. Depending on how hard you press the foot  brake or hand valve, the pressure in the service  line will similarly change. The service line is  connected to relay valves. These valves allow the  trailer brakes to be applied more quickly than  would otherwise be possible.  Emergency Air Line. The emergency line (also  called the supply line) has two purposes. First, it  supplies air to the trailer air tanks. Second, the  emergency line controls the emergency brakes on  combination vehicles. Loss of air pressure in the  emergency line causes the trailer emergency  brakes to come on. The pressure loss could be  caused by a trailer breaking loose, thus tearing  apart the emergency air hose. Or it could be  caused by a hose, metal tubing, or other part  breaking, letting the air out. When the emergency  line loses pressure, it also causes the tractor  protection valve to close (the air supply knob will  pop out).  Some vehicles have "dead end" or dummy  couplers to which the hoses may be attached  when they are not in use. This will prevent water  and dirt from getting into the coupler and the air  lines. Use the dummy couplers when the air lines  are not connected to a trailer. If there are no  dummy couplers, the glad hands can sometimes  be locked together (depending on the couplings). It  is very important to keep the air supply clean.  Older trailers do not have spring brakes. If the air  supply in the trailer air tank has leaked away there  will be no emergency brakes, and the trailer  wheels will turn freely. If you crossed the air lines,  you could drive away but you wouldn't have trailer  brakes. This would be very dangerous. Always test  the trailer brakes before driving with the hand valve  or by pulling the air supply (tractor protection valve)  control. Pull gently against them in a low gear to  make sure the brakes work.  If you do cross the air lines, supply air will be sent  to the service line instead of going to charge the  trailer air tanks. Air will not be available to release  the trailer spring brakes (parking brakes). If the  spring brakes don't release when you push the  trailer air supply control, check the air line  connections.  When coupling, make sure to couple the proper  glad hands together. To help avoid mistakes,  colors are sometimes used. Blue is used for the  service lines and red for the emergency (supply)  lines. Sometimes, metal tags are attached to the  lines with the words "service" and "emergency"  stamped on them. See Figure 6.6  Glad hands are coupling devices used to connect  the service and emergency air lines from the truck  or tractor to the trailer. The couplers have a rubber  seal, which prevents air from escaping. Clean the  couplers and rubber seals before a connection is  made. When connecting the glad hands, press the  two seals together with the couplers at a 90 degree  angle to each other. A turn of the glad hand  attached to the hose will join and lock the couplers.  2005 Commercial Driver’s License Manual  6.2.5 – Hose Couplers (Glad Hands)  6.2.3 – Trailer Air Supply Control  6.2.4 – Trailer Air Lines  Page 6-5  Section 6 - Combination Vehicles 