PastorMatt, what is happening is that the gas is expanding in volume to the point is extends up the filler tube and gas pours out through the vent in the gas cap. There is no pressure build up per se.

I had the same thing happen to me with my '51 and it occured inside a very hot Arizona garage. Right next to the electric water heater! I never top off the tank unless I am on a road trip.

Kent

1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix