[Tommy]

Gasoline (and other liquids) boil when the combination of temperature and pressure is right. Higher pressures and lower temperatures inhibit boiling. That is why vapor lock is not an issue downstream of a functioning fuel pump under normal underhood temperatures. Fuel may boil in a carburetor float bowl, but that is not "vapor lock" and fuel in the float bowl is not pressurized.

Vapor lock occurs at the fuel pump when the combination of pressure and temperature of the incoming fuel is right. There are two reasons why the temperature of the fuel may be high enough to boil. One is a very high ambient temperature in the fuel tank or line to the pump. Ethanol makes that more possible, but it is still very unlikely. The second reason is in the design of the pump's internal pressure control system. On systems without an external pressure control and fuel return line, the pump has a spring controlled valve on the output side of the pump that closes the flow of fuel to the carburetor when it reaches the designed pressure (e.g., 8 PSI). To keep from stalling the rotating pump vanes, the fuel is diverted through the pump housing back to the intake side. That means that the same fuel will be pumped through the vanes repeatedly until the output valve opens. As electric fuel pumps get hot while operating, that fuel will get hotter with each pass. When the temperature gets hot enough it will boil and the pump will become vapor locked as the vanes cannot pump vapor effectively. This phenomenon is why an external pressure relief valve and return line helps. It keeps fuel from getting cycled repeatedly inside the pump housing.

The pressure of fuel flowing to the fuel pump can also cause vapor lock. As the pump's vanes grab incoming fuel and push it out toward the carburetor, it creates a low pressure area at the intake. The combination of ambient air pressure inside the tank and gravity from the weight of the fuel in the tank and above the pump forces fuel to fill that low pressure space. If the fuel pump is mounted too high relative to the fuel level in the tank, the pressure from gravity will be reduced. This reduced pressure means the fuel can boil at a lower temperature. So with a combination of high ambient temperature and low pressure at the intake of the pump, vapor lock can occur.

The observation that your problem becomes more pronounced as the fuel level in the tank comes down is what lead me to suspect that your fuel pump is mounted too high. Either that or it is failing and unable to pump efficiently without help from the weight of a full tank of fuel.