I feel the same way, if I unhook that rubber tube from the top of the large metal one the engine will quit right away. The truck had a lot of equipment welded to the bed at sometime in it's life. Superior Aluminum Body out of San Francisco did the work in '88. It has the stickers on the doors. The whole truck was painted bright orange at one time, I mean the whole truck inside cab, bed, under frame (not overspray but a shiny coat). Could be from a mine or ???? To me that metal tube to the crankcase has got to be restricted in some manner, like you say if wide open the truck should stall.
