Thanks Charlie for starting this thread.
I bought a 74 C20 2 weeks ago. I've been working on it more than driving but so far, so good
I went to put gas in her and about 5 seconds in the pump shut off and puked some gas onto the ground. Ultimately it took about 20 minutes to put 19.2 gallons in because I had to hold the pump lever to a trickle.
The truck has 2 tanks but the driver's side is disabled as the filler neck is removed.
I have a 2004 C5 Corvette that shuts some pumps off early, which I will soon fix. So I have been researching the basics of air displacement into an evap canister, and a clogged hose will prevent displacement and cause the pump to shut off early.
The C20 doesn't have an evap canister, tho it must vent somewhere (sorry environment). The filler neck has 2 ports, a main port straight down and a smaller side port. Each has it's own hose but then combines prior to entry to the tank. I'm hoping this secondary port isn't the displacement breather since it would vent back into the filler neck and clearly be an issue with a fancy gas pump nozzle. I am hoping that the displacement breather comes off the sending unit and whatever home made set-up done before is causing the issue. I haven't taken a good look at the sending unit to know whether it's a 1,2 or 3 port. If you know if this is where it's supposed to vent, please post.
Like Delta, I'd like to know what hose size the Dr. is using for at least a workaround.
I plan on posting more when I know more.