The cause of this problem is actually a bad carbon canister. The gas tank on an 80 is not vented to atmosphere, because of smog requirements it vents through a canister of activated charcoal. When the carbon canister gets saturated or clogged it no longer allows the tank to vent and it pressurizes and forms a vacuum based on the outside temperature. With repeated cold and heat, the tank cracks in its most fragile location. Notice the venting sound or vacuum hiss when you open the cap to fill up with gas??? There you go. Replace your carbon tank (see other forums as these are no longer available from Toyota, but there are a number of aftermarket solutions designed for other vehicles that run between $50 and $100).