Turns out the springs were indeed the most likely culprit!
The return spring was present but both springs were extremely worn. Rebuilt the thing again and put in some new springs (and took some wire wool to the pump well for good measure) and everything seems to work as it should. Not back on the truck yet but I put some fuel in the bowl and I can already see a MASSIVE improvement in the accelerator pump function.
Aaron V. 1948 Chevrolet 3800 216/4-spd 1979 Chevrolet C30 Ramp Truck 454/TH-400 1995 Jeep Cherokee 4.0/5-spd 1970 Ford F100 390/4-spd 1973 Ford Maverick 302/C4 2004 Honda Shadow 1100
"While driving yesterday I saw a banana peel in the road and instinctively swerved to avoid it᠁thanks Mario Kart."
