Pool Leak or O-Ring Issue

[ozbrown]

Hi,

I hope someone will be able to provide some insight as to which way to proceed.
Set-Up
1 Pump for Filter/Clorinator - This is fed by a 3 Way Jandy Valve which normally operates on the pool main drain but is able to switch to the spa drain.
1 Pump for In-Floor Cleaning jets - This is always fed from the main drain.
The Main Drain and Skimmer connect I think and then go to the main pump/cleaner pump. I may be wrong as the prior owners didn't provide a plumbing plan to work this out.

The issue
If I run the main pump (while it sometime loses prime overnight and the spa which is higher than the pool sometimes drains into the pool) generally it is able to run without any issues. However when I turn the cleaner pump on simultaneously it progressively makes the main pump lose prime. My question is: Is this likely to an issue with the Jandy Valve or Linked to the plumbing poolside?