I just had rain and was trying to get the main drain to suck up some dirt from brushing, and even with float valve diverter closed I was still not getting full suction. I just figured out a trick to do this. All you need to do is hold the Weir door closed until the skimmer drains. The float valve will close and voila! Full main drain suction. After you are finished, reset the float valve by pulling it out and re-seating it and you are back to skimming. Hope this helps someone!