Success \o/ The spa water is no longer draining into the pool when both pumps are off (drawing attached) - not even a fraction of an inch. I had to:
1) Replace the internal flapper assemblies of all 3 checkvalves with
authentic Jandy replacements. The
generic ATIE versions did not fully seal. Check valve #1 (after the filter) was *completely* busted - plastic had worn away, the spring had no tension (it was just flapping), dried glue on the inside of the pipe, etc. Picture attached.
2) Replace the internals of the Pentair 3 way diverter valve V-3. After many years, this part seemed worn away and probably wasn't holding a great seal (picture attached). There was also some dried glue I had to remove with sand paper.
3) Replaced the NSI Tork Actuator on V-3 with a new Pentair actuator. I suspect it wasn't closing all the way, and it was quite old. Perhaps I could have adjusted the internals but I instead just purchased a Pentair actuator that I figured would be designed to better work with the Pentair diverter valve it sits on top of.
Thanks so much
@Jimrahbe &
@Poolbreh for the insight about the problematic plumbing & multi-pump design that I inherited. I will be installing a Black & Decker 3HP VSP over the coming weekends and I'll seriously consider replumbing to one of the single pump designs as suggested by
@Poolbreh once I feel more confident in my pool/PVC/plumbing skills.
You all rock. I've joined as a Gold Supporter in appreciation of you valuable people and this valuable forum. Thanks again!