I have the same leakage and the o-ring is 7.5x1.5mm.
Google for "Mikuni Carburetor O-Ring" or something like that... but you will find the size for small money.
Anyways I measured the T joint. 7.5mm inner dia and housing is around 9.45mm - so 7.5 with 1.5 thickness will fit, maybe a 7x2mm will also fit and be leakfree but no need cause 7.5x1.5 is available.
Hope it helps... I know that other bikes has same carb fuel pipes in use but didn't found a solid part number.