Does it start out on full? Mine does the same thing but it only goes to about 3/4 when it is full. I have new sending unit. ???
The vent on mine went to the canister next to the radiator. I removed the canister and put a fuel filter at the end and mounted to my inner fender, should be higher than the fuel can get when tank is full. I have had no problems with mine. (72 duster but should be the same)