I've searched past threads, and am having trouble figuring this one out as well, for my situation at least. I have a waterfall spa setup (spa 2' higher or so than the pool water level).
I run pool-suction and pool-return, and have a 1/2" aux line that goes from pool-return to spa-return to push water into the pool. It has a check valve + a manual on-off valve (as sort of a redudancy).
I moved to a new pump which I run at a lower speed, so I assume I have less flow to overflow back into the spa. Now I notice the water level drops overnight and during pumping does not quickly regain lost ground.
I turned the manual on-off valve off to bypass the check valve flow, pumped from pool --> spa (to start at a high level), and still get quite an overnight drop in the water level of the spa.
Does that leave my grey three-way jandy valves (on the suction and return) as possible candidates as cause for this trouble? I opened them up this morning before I made this post to inspect, and the diverters 'looked' OK. The O-rings seemed fine (except one of the small O-rings on one had been split). This is the o-ring that comes between the handle and the diverter (not the outer o-ring). Not sure how to tell if the diverter itself is shot. The square rings seemed to have a little height to them, and the diverters are a black color. Not sure if they are never-lube retrofits or the originals.