John RR is correct. Are you sure the gas is coming out the end of the vent hose? My 69 Roadrunner came from the factory with a leaking vent tube at the tank due to bad sealing. When I bought my 69 GTX in 1990 it had the same problem, couldn't put in more than half a tank of gas. Ma Mopar fixed my original Roadrunner, and I put a new tank in the GTX.