More likely the crud was condensed mositure in the tank that settled around the cluk, which was also probably the low point in the tank. The high humidity levels were already mentioned, drawing moisture in from the vent. As in full scale, water will "sump" in a gas tank and remains separate from the gas. It generally has a cruddy brown appearence but that color could change with some of the oil colors out there.

In a three line tank, where the fill is used to de-fuel, you need an additional clunk to keep the fill line in the fuel. Otherwise you won't be able to remove all the gas from the tank. That's one of the problems with a three line, the extra clunk, which has been a problem at times interfering with the primary fuel supply line. Three line systems look neat but truly don't do any more than add complexity over a two line system.