Probably not entirely useful to you anymore, but in case someone else has the same question: the hose from the top rail of the carbs is a vent hose. It's routed straight up and then lays down the back of the air box.
And whenever my carbs are drained it's easiest for me to refill them (at least partially) manually before connecting the fuel line to the tank. This way I don't have to suck on the vacuum hose or wait / wonder about them getting filled.