I got confused with this thread and apologize. I missed the original issue of bubbles and concentrated on the pump basket draining down. If you have air bubbles coming from the return jet(s) you have a suction side leak which could be a leak at the pump basket lid. If the pump basket is draining down overnight then the problem could be a leak in the multiport valve or the basket lid itself. Reading this whole thread again it seems the leak is most likely at the pump basket lid gasket.
Again, sorry I missed that.
Al