Glad hands are the coupling devices at the ends of the rubber air hoses that connect a tractor to a trailer. They use a simple twist-lock design with rubber seals to prevent compressed air from escaping. There are always two connections: a blue line for service brakes and a red line for emergency brakes. When you press the couplers together and twist them, they lock securely.
Keeping the rubber seals clean and intact is critical for safety. Cracked or dirty seals leak air, which drops the system pressure and can cause the trailer brakes to lock up automatically. Drivers should inspect these seals during every pre-trip inspection. When the hoses are disconnected, glad hands should be snapped onto dead-end mounts on the cab to keep dirt and moisture out.