Two connections must be made to join a trailer’s brake system to the brake system of the towing vehicle. One connection carries air to fill the trailer’s tanks and is called the supply or emergency line. The other carries pressure for the service brakes and is called the service line. Two pairs of metal trailer couplers or ‘gladhands’ are used to make these connections. Each coupler includes a flexible seal that provides a leak-free connection. These couplers must be kept clean and in good condition.
Most towing vehicles have receptacles called ‘dead end’ or ‘dummy’ couplers that protect the couplers from dirt and water when not in use.
To connect the couplers, the surfaces that contain the seal are placed at an angle against one another. Rotating them into alignment completes the connection. Once correctly joined, the connectors lock together and considerable effort is needed to pull them apart.
Since there are two air lines to the trailer and the design of the supply and service couplers are basically the same, there is a possibility of cross-connecting the lines. In most cases, the trailer couplers are colour-coded during manufacturing — red for the supply line and blue for the service line. By matching these colours, the lines are properly connected. To further prevent cross-connections, most couplers are designed so that they will only fit onto the matching coupler. Some couplers, however, have no features to prevent cross-connection.