I have a C150L (1972) and the vent leaks when the left tank is full. A&P and checked and can not locate why it's leaking. Only leaks about 1/4 to 1/2 gallon before it stops.
Check the fuel filler where the neck is welded to the tank. A crack often develops there because the fuel guys rest the fuel nozzle in the filler which puts a bending force on the weld and results in a broken weld.
There is a vent valve located on the vent line of the left fuel tank. The valve is a hinged flapper type that, if not installed properly with the hinge at the top of the valve. will leak. Unfortunately the valve is located on the vent line inside the tank that is difficult at best to access. Fuel tank removal is the ideal way to troubleshoot or replace this valve...
