2005 Commercial Driver’s License Manual If you do cross the airlines, 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 airline connections. 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 airlines, 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. Some vehicles have "dead end" or dummy couplers to which the hoses maybe attached when they are not in use. This will prevent water and dirt from getting into the coupler and the airlines. Use the dummy couplers when the airlines 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.