Try our new info resource - "Aladdin's Cave" (Main menu)Just added a separate link to Ash's Dropbox thread (shortcut)
That was not an easy fault to find if I understand you correctly the brass overflow tube is leaking where it fits through the base of the bowl.
I am in the process of re-assembling my CB00/4 carbs and the float bowls were going to be fitted today. I came on to the forum to see if I could establish (beyond doubt) which way the main jets go in and saw this article. Timing could be good for me as three of the four carbs were leaking fuel out of the over flow pipes when I started this project. Assumption was that the floats were stuck in the gloop at the bottom of the float bowl (and there were signs on the floats this could be the case). I will now do a quick check around the overflow pipe/bowl joint.