Here is a picture of the original cap. New caps come for places in Asia. Construction is unknown and fit is problematic. The original AD fuel system must vent when the vacuum inside the tank becomes too great. I believe there is a check valve in the cap, but I really don't know. A fuel pump pumps gas to the carburetor at about ~3.5 to 4 lbs. If you didn't have a vent, the truck would starve for gas just like a lawnmower with a plugged-up vent hole. Pressure (not vacuum) build-up inside the tank is another matter and for the AD trucks, this is a weakness. The gas cap needs to prevent gas from leaking out but if the pressure is too great it overcomes the seal(s) and leaks. The solution is to allow an area for the gasoline to expand by not filling the tank too full. Another issue I heard about is that some Asian gas caps don't fit tightly and allow gas to leak regardless of pressure.
If a vendor sells you a part, that was made for your truck, that doesn't work well or doesn't fit, contact the vendor and explain.
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
