Holley 1920 carb issues!

The hard starting is likely too hot a carb or gas line. The newer gasolines have a lot higher volitility, so they boil more easily and turn to vapor in the carb or lines when the pump pressure is gone. As mentioned, the a thicker carb gasket may help, and keeping the lines cooler. Also, if the fuel pump's output check valve or the needle valve in the carb are leaky, then the pressure will bleed off and the fuel will boil more easily. A good fuel pump and needle should hold good pressure to the carb input for 30 minutes or more, and that should help some.

Also, see if you can find non-ethanol fuel and see if that helps. Alcohol boils more easily.

If the 'yellow button' is on a lever on top of the bowl, then that is the bowl vent area, and fuel should not come out of there. If so, then the needle is not seating well and fuel is coming out of the vent. Take out the needle and seat and clean them. Have a pix of the 'yellow button'?