1975 dart sport gas tank vent blocked?

Will try to explain the vent tube on the 1974 thru 1976 A bodies. The vent tube's total length is about 9 inches and it runs close to the inside roof of the fuel tank. The end of the tube is crimped shut and it is tack welded to the roof. Now as to how it vents gas vapors. Following the tube to the 6 inch mark, you'll find a very small 3/64 hole in the top of the vent tube. If you try blowing hard into the tube,it appears to be blocked. Try blowing gently with your mouth and you should feel pressure being released. We used a Mityvac pressure tool with gauge to prove the tube wasn't blocked. Unfortunately the only way to actually see the 3/64 vent hole would be to cut the top of the tank off and separate the tube. There's no description of this in the service manual. In 1976 Federal law required the use of rollover valves, which were installed outside of the fuel tank.