Gas Tank Pressure Build-Up

[marvincorty]

I think there may be a problem with the gas tank cap on my CA78. After fueling with gas that is obviously cool and then letting the bike sit, pressure builds in the tank to a significant level. It gets high enough to force fuel past the carb float valve and cause the float bowl to overflow. I know the cap needs to vent inwards so fuel will flow to the carb but is this supposed to be a one way process or should it vent both ways? Has anyone else run into this same problem? Maybe the cap vent is not clear and I need to try and disassemble it if that is possible. Suggestions or ideas?