I am far from an expert on Goldwings but they do have an electric fuel pump so if it is just sitting then no gas is getting to the carb from the tank no matter how full the tank is. So if the manifold is filling up I would check not only the float but also the needle and seat because it could drain what gas is left in the line into the carb even if the float is in the proper position.
OP mentioned this occurring on a full tank, which can gravity-feed the carburetors even without the pump running...
Agree with this.
on the 1500s, if the fuel tank is full to the cap or nearly so, and the Fuel Petcock Shutoff valve has a bad diaphragm ( pinhole ) it will NOT shut off, and gas flows to the carbs by gravity feed.
And if the floats are not properly set, or the bike is on the sidestand, they won't prevent it from getting flooded.
Here's a pic of the location. Right above the word 'Gas'.So it has a Petcock? Where is it located?
The petcock is above any possible gas level even with a full tank so unless the law of gravity has changed recently how would a full tank be different from an empty tank insofar as the gas would still have to go UP before it could go down? And you will not have enough flow to start a siphon. As a matter of fact, if you had a hole in the diaphragm then I would think it would pull air from the carb back to the tank before the other way around. I guess it is very remotely possible that it might pull gas to the carb from the drop in the line to the carb but I think if you measure and do the math the level of the gas up to the petcock from the tank and then down to the carb is just not going to support siphoning.
Here's a pic of the location. Right above the word 'Gas'.
Kit to rebuild it: https://cyclemax.com/inc/sdetail/gl1500_fuel_petcock_rebuild_kit/4102/20457
One way to find out, take the gas line off and see if gas comes out of it. At least more than what is just between the line and the petcock.