What is a 'glad hand' on a combination vehicle?

Glad hands are the coupling devices that connect the service line and emergency (supply) line air hoses between the tractor and trailer. They have rubber seals (grommets) that compress to form an airtight connection. The two glad hands are typically color-coded: red for the emergency/supply line and blue for the service line, often with shapes or labels to prevent cross-connection. Always check the grommets for damage before connecting; a torn or missing grommet causes air leaks. After connecting, listen for air leaks and watch the trailer air pressure gauge to confirm the trailer is being supplied. Always disconnect by pulling the glad hands straight off, not by twisting, to preserve seal integrity.