The pump lid o-ring is usually the culprit.

Yes, it’s almost always the pump lid o-ring or a suction-side air leak. Fix that before you do anything else.

When a skimmer loses prime, it’s because air is getting in on the suction side of the pump. The pump can pull water but loses suction over time. The lid o-ring is the most common leak point — it dries out, cracks, or gets a speck of debris under it. Remove the lid, clean the o-ring and the groove, lubricate with silicone pool lube, and reseat it. If the o-ring is hard or cracked, replace it.

Other quick checks: Make sure the water level in the pool is at least halfway up the skimmer mouth. A low water level pulls air. Also check the weir door (the flap inside the skimmer) — if it’s stuck closed or missing, the pump can pull air instead of water. And confirm the skimmer basket isn’t completely clogged, which can starve the pump and let air in.

If none of that works, you might have a hairline crack in the pump housing or a loose union fitting. But start with the lid o-ring. Nine times out of ten, that’s it.

Don’t overthink it — lube the ring and move on.