The petcock just lets the fuel flow to the carbs when vacuum is applied to it, it doesn't control the fuel levels in the carbs; that is up to the floats and float valves. If the levels are set correctly, and the floats actually float and shut the valves, the petcock could be bypassed and the carbs still would not overflow. So, it comes down to, the float valves are not shutting off. The question is, why? There could still be a bit of grit in the valve seats, or the floats could be sinking instead of floating. The only way the petcock could be at fault is if the vacuum diaphragm is leaking fuel into the vacuum port, which would usually show up as fuel in the cylinder and oil, not leaking out the carb overflow.