Al, I usually trade off some capacity in order to minimize losses and keep the sample as clean as possible. Try over-threshing the crop if possible and run lots of air. That means tighter concaves and faster rotor speed than most recommend. This will help minimize the return volume and help keep unthreshed pods out of the tank because there are simply fewer of them. Of course, when cutting crops for seed, this might not be the way to go. In your case, I doubt that you can get green bean pods threshed, so letting them ripen and dry down would help if that's possible. If you need to cut now and you have green pods, cover plates are an excellent investment. It is impossible for the lower sieve to keep unthreshed pods out of the tank because they are nearly the same size and density as the beans.