Air leak. What is this? Very old polaris pressure vacuum diverter?

[syntaxerr]

Hi All-
I recently purchased a house with a pool built in the 90s. I had a local pool company come do a tour of the equipment and "everything tested out". With this said, I am maintaining my pool and everything is looking great with respect to water clarity, chemistry, filtration. Props to this board.

My only issue now is that I have an air leak and the spa will partially drain to the pool each night. No visible water leaks, however when my variable speed pump turns off, I hear some air enter the system around this Black polaris plastic diverter valve. Can anyone ID this for me? Right of the Jandy in the picture. What is this for? I thought it was a spa makeup valve but I don't think so now.

Thanks!

 

[charlieTX]

It looks like it could be a space saving joint connector with three unions to make repair / upgrade easier than cutting glued pipes and couplers.

Best guess would be the O-rings around one or more of the unions are done and need to be replaced and lubed, grab an oil filter wrench to get them off.