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North UK

If it has been 20 years.  the rubber and mechanisms inside the Mikuni vacuum pump would be bad due to old age, just nature of rubbers.

I would replace the vacuum pump.

Since it is a GV – No fuel should come out of the “TANK” when tap is set to  “OFF” mode.  If it did, the pump  will slowly drip fuel to the carbs , then eventually as you suspect, the floats will get jammed , then bike won’t start due to fuel inside cylinders  (hydrolocking) ,   also, it might damage the crank and snap a rod or bearing failure.

I’ve seen intake rubbers go bad and they are extremely difficult to acquire nowdays.

This is assuming your bike has a “vacuum” pump   –  Not the  “ProtoType Electric” one Hyosung was using on 1999-2004 era GV’s  (They are rubbish) , Mikuni Vacuum pump would be better and it doesn’t need electrics.

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