So here's the latest on the mystery of our pump air leak:
2 days ago we opened the air bleed on the filter and ran the pump until the air stopped hissing and water was squirting out, then we tightened down the bleed valve and things looked good.
Yesterday, I noticed the water level in the filter basket was low again (maybe 2" below the lid) so I repeated the air bleed process and ran the pump a few hours. Hubby checked on it later in the evening and said water in basket was low again so he bled the air and then shut the pump off after looking for any obvious signs of leakage.
This morning: the filter basket is still full of water after being left in the off position overnight
So it seems that the air is entering the system mostly while the pump is running? What would be some likely suspects in this case? The skimmer is plenty full so it's not likely to be sucking in air there, we thought it might be a leaky vacuum hose so we disconnected the vacuum hose and closed the vacuum port. Water level of the pool is not dropping noticeably faster than by ordinary evaporation.