There is a screen in the tube right below the tank, we took it out on both the 1440 and the 1660, I assume the 1680 would have it too. We didn't have any problems after taking it out.
We had the same problem in our 1680 and the screen was already removed. We had a bacteria growing in a couple of tanks. looks like a black stringy, slime that you will see when changing filters. If this is the case, talk to your supplier and get some additive to kill it. You'll probably have to drain the tank also.
we had the same problem. It was caused by the shut off valve right before the filter on the side of the combine. It had a small needle type valve aand it would not let the chunks of rust through to the filter. WE took it our and put in a ball vavle so the opening was bigger.