Possibly have a leak in which air is coming into the plumbing, but I'm not sure. This is an older pool with old equipment. 2 floor drains, 1 skimmer, 2 returns.
About five days ago, the water level was getting low and since the pool would be closed soon, instead of adding more water, I decided to circulate water from the two floor drains only. (Possibly my first mistake.) After I first turned the valve to "floor drain only," it seemed to be working. But then a few minutes later, I noticed that PSI went to 0 and the pump basket (I think that's what it's called) was not full of water and the filter water was getting lower. So, I turned it off, we primed it and got it working again, running off of floor drains only.
A few days later, it rained and our water level went back up. Wanting to use the skimmer basket to collect leaves again, I (rather quickly, sorry to say) moved the valve from "drain only" to 50/50. Initially again, it seemed to be working, but upon checking the same situation had occurred -- 0 PSI, empty pump basket, filter losing water -- and no suction in the skimmer. So again, I turned off the pump, and attempted to prime and restart. The problem is, this time it seemed that the pipe coming into the pump from the floor drains would not fill up. I ran water from our hose into the pump basket for close to five minutes, and I could see water running down the pipe toward the floor drains.
I'm not the most patient person, and I was getting worried ... so I moved the valve to "skimmer only," primed the pump, and restarted. Everything was as usual -- a few small bubbles from the returns (normal in skimmer only for this pool), normal PSI, filter and pump basket filled with water. I let it run overnight in this position.
Today, I tried to s-l-o-w-l-y move the valve back to 50/50, but every time it gets past "skimmer only" and opens the main drain valve, it sucks the air out of the system. I've tried priming and restarting, etc. but the problem remains.
My conclusion is that I either have a ton of air in the system between the main drains and the pump, which I'm too inexperienced to know how to fix ... or there's a leak somewhere between the main drains and the pump. I haven't done a leak test yet -- that's next on the list.