References for "glad hand" in its common meaning of "effusive but perhaps insincere greeting" seem easy to find , and it's easy to find definitions for the truck-hose-coupler meaning of gladhand ,  but I'm at a loss for finding out the origin of the latter usage (aside from imagining gladhand might derive from a manufacturer's name). Is any evidence available?
My conjecture is that a "glad-hand coupler" derives from the definition of "glad hand". From Dictionary.com, one definition is:
to greet others with enthusiasm, especially feigned enthusiasm: The candidate spent weeks glad-handing around the state.
The Wikipedia entry on semi-trailers explains that a glad-hand coupler (emphasis mine):
"Glad-hand" connectors (also known as "palm couplings,") are air hose connectors, each of which has a flat engaging face and retaining tabs. The faces are placed together, and the units are rotated so that the tabs engage each other to hold the connectors together. This arrangement provides a secure connection, but allows the couplers to break away without damaging the equipment if they are pulled, as may happen when the tractor and trailer are separated without first uncoupling the air lines.
If you go "glad-handing", you are making connections that are insincere. I think that because the couplers are attached but easily breakable, they are called "glad-hand connectors" because they are physically "insincere".