#3 Bowl vent - leaks on/off & removed VM26ss

Help, at a bit of a loss.....

Took carbs off to rebuild & bench sync. This #3 carb has off and on left it's mark on garage floor. Just thought stuck float valve.

In clear tube float level jig she leaked. Rebuild all 4, actually surprised at how clean all was but cleaned/cleared passages/blew air/replaced float valve's and redid clear tube check(perfect levels 2nd time ) but #3 vent still leaked.

Took bowl off and filled 1/3 full no leak, filled 1/2 full no leak, filled 1/8 below top and slow lil gas bubble forms at bottom of vent.

Air pressure in top of vent with bottom plugged... NO Air leaks.

WHAT THE HECK IS CAUSING THIS ????

If not already done, would inspect for possible crack in the overflow tube.

Good Luck!