I have a pool /spa combo where the spa water feeds the pool. The last few weeks, inconsistently (as it seems to happen every few days) the water in the spa drops alot after I have run the filter. The pool is fine. Based on what I am reading online, it seems like a check valve issue. Please let me know, as per the pics, which one I am suppose to look at (i.e. I seem to have both a black jandy valve (next to the skimmer & pump) and then a white jandy valve (as per the pic). I also noticed now that the black valve does not seem to have a knob on it (not sure if that can be a issue). Thank you, Gary