I've recently been having this problem with the pool pump. We replaced the pump and filter and added a salt water chlorinator about five years ago. And replaced the inlet plumbing and skimmer last year. The pump ran fine last year and this year up until now.
What's happening is the pump draws too much water and no matter how full the pool is, It will over pull from the skimmer and the water level collapses and it draws mostly air. The skimmer is the only entry point to the pump. When I hook up the vacuum hose the pump primes just fine and runs at 10psi through the filter. It also over draws when discharging/flushing out the filter.
My thought was that there isn't enough back pressure through the filter and I flushed and added fresh D.E. powder but that didn't help. I even over added thinking maybe I didn't have enough. And my next move was to open up the filter and inspect it and then refill it with powder, but the fact that it's running at 10psi through the vacuum hose makes me think that won't help.
Could it be that the pump is set at a (too) high flow rate? Is there a way to regulate that? And how would that change on it's own?
If anyone has experienced this, I'd appreciate hearing what you did to resolve it or if someone has in-depth knowledge and can explain what's happening and how to fix it, I'd appreciate as well.