I think the 3 way tee shown in Bill's diagram with the line going back to the rear and up behind the generator mount is actual a line for an air compressor on many of the larger trucks.
I did some more snooping with the added info you guys posted above and here's a screenshot from a 1963 Master Parts Book indicating the model applications for the 3 way tee.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
