no check valve.
You seem to have a vent system that is older than your car. up through 1970 the vent was simple and ended inside the rear frame rail. it is actually design not to vent anything out of the tank, but rather let air into the tank to balance the fuel being pumped out. 1970 CA cars and cars after that have a complex system of tubes that route the vented vapor to charcoal canisters and such. Does your tank have 4 vent tubes that have been plugged (driver's side toward the top of the tank) or do you have a 70 and earlier tank (no vent tubes on the tank)? The other odd thing is that pressure built up in the tank so that when you opened the cap it expelled gas. If your vent tube was functional all the overflow would have come out the vent tube into the frame rail. it has to be plugged somewhere...
1971 and up tanks have a separate "tank within a tank". It can only be accessed with the fuel tank removed. If you have a later tank and the system is not functioning properly, that inner tank (part of the emissions system) could be leaking from the rubber plug on the top of the tank. You can only get to that plug by removing the tank from the car.