strange possible suction-side leak

[satpoolguy]

I ran into a weird situation I'm hoping for some help on. This morning I turned my pool on to spa mode and ignored it for about 20 minutes. When I returned, the spa was completely empty and the pump was running dry. I then noticed that the return valve was still set to split the return water between the pool and the spa, which explains the spa being empty (that valve is not working at all, but that's not the issue I'm posting about). Now when I run in spa mode, the pump loses prime. Sometimes it pumps for a few seconds, and sometimes it immediately loses prime. It works just fine in pool mode, so this seems to point to an issue with the spa drain plumbing. But things worked well enough for it to pump all the water out of the spa, which is confusing. I put soap suds all over the exposed spa drain plumbing and didn't see any get sucked in. I removed the valve and everything inside looks fine. My theory is that I have an underground leak but I've heard these are very rare, plus everything had been fine just a little bit earlier when the spa emptied. Any thoughts or suggestions?

 

[kimkats]

Here is a bump for more eyes.

 

[satpoolguy]

So I think the issue is that even after I let a fair amount of water flow from the spa drain to the pump (i.e., pump lid removed, just letting water flow out), there was still quite a bit of air in the piping. I did this again, and after a steady stream of water, a bunch of air came out. I continued to let water flow and got a few more smaller bursts of air. I put the cover back on and ran the pump, and I got a few very small bubbles (not enough to disrupt the prime) and after a few minutes those stopped too. I'm inferring that the turbulence when refilling the pipe left a bunch of air in there, and it's now been purged. I'll monitor it, but I'm cautiously optimistic my issue is resolved.