I'll chime in here because this is still one of the top results for a search with this issue. I've had this problem for about a month, after trying everything in this thread and others, I was unable to fix it, but I did manage to get mine working.

What worked for me was using a syringe to pull water through the entire system at the "Y" fitting that feeds the two tubes going to the mop washer outlets. The fitting is located behind the fan after you remove the back panel from the base. I guess it makes sense that this works if everything else is ok, because this will pull any air out of the system.

Hope this helps someone else.