Air brake glad hands are coupling devices used to connect the service lines (blue) and emergency air lines (red) from the truck or tractor to the trailer. The glad hand couplers have a rubber seal, which prevents air from escaping. The rubber seals in the couplers need to be clean to function properly when a connection is made.
When connecting the truck and trailer together the glad hands must also be connected to allow air to activate the trailer brake sysem. Phillips, Haldex, Sloan and Tectran couplers are connected by pressing 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.
Drivers must make sure to couple the proper glad hands together. To help avoid mistakes Phillips, Haldex, Sloan and Tectran air brake glad hands are color coded. Blue is used for the service lines and red for the emergency lines. Metal tags are sometimes attached to the lines with the words "service" and "emergency" stamped on them.