Yes, that is what I suspected, that somehow there is air going into the suction pipe. Will check that. Mind, you might be difficult job as most of it is underneath the deck. Or I might change all the pipework going from suction pipe to pump and beyond. What you think? Momentarily it's a bit of a mix of pipes and honestly, a bit of a mess.
The bottom drain is OK, I never used it yet, but if I need it is there, and will just run out of the pipe in the pump room, near the sprinkler pump.