The way it works is that when the engine is turned on a vacuum is pulled and the diaphragm shifts to one side allowing fuel to pass from the tank to the carbs, which is why if you have your petcock set on prime fuel flows whenever but when set at normal or reserve no fuel should spill from the fuel tank. So if the vacuum valve is knacked then fuel will pour constantly, it seems as if it is sticking if it is ok when you reassembled the bike but then after starting the engine pulling an initial vacuum they don't return to the original position when stopped. Try replacing the petcock at the base of the fuel tank where the vacuum is pulled to let fuel out.
Hope you get the bike right soon.
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