The valve in the filter base is the bypass so oil can go around the filters with cold thick oil or if filters plug. I for sure would want that valve in there and it may well be just kind of hard to get a hold of to get it out. But, without it, the oil will go right on around the filters.
On the drain back valve in filters, you can look in the bottom end through the holes or slots and there should be a diaphragm and also a tube in the center.
If filters and base check out, I would be checking the suction tube. Might be gasket failure where it is bolted to front cover.