leaking carb

I've rebuild many carbs before, but never had a problem like this. I took carb apart cleaned it, rebuilt it, new needle and seat, seals, float. I adjusted the float, put it all back together and stills have a slow leak. I've read about ethanol eating rubber and aluminum away, so I replaced the bowl and it still leaks. Rebuilt the carb again with another float and still leaks. I've heard ethanol distorting the vapor release in the gas cap causing pressure to build allowing slow leaking. So I loosened the gas cap and still have a slow leak and what I mean by slow leak a drop or two every 3-4 minutes. I've noticed the replacement floats are plastic, is this the problem? Has anyone have this problem?