I think I know the answer to this one, but I'm really hoping I'm wrong.
Have an inground pool, single skimmer, no main drain. 2 speed pump (not sure on make/model) that came with the pool when we bought the house. For 2 years, everything has been working well for the most part at least with this (everything else is just about brand new), but every since opening in the spring, I just don't get the same suction out of the pump as I used to. I used to be able to vacuum even on the lowest section, but now on high, it constantly looses prime. On low, I can't even circulate the water, the pump basket just slowly fills up with air. I can also hear air moving around in the top of the sand filter.
I've taken apart the pump, cleaned out the impeller, regreased all the Orings, and done a soap test around the basket, with no leaks that I can see. At this stage, I can think of a few options:
1) Most likely (but hopefully not) I think I might have a suction side leak, and since I've checked all the obvious places, it might be underground...
2) The pump is starting to just not work so well, its an old pump, but I don't want to run out and buy a new one just to find out it is a leak somewhere. The thing that is throwing me off is the fact that on low, the pump basket fills up with air, even on high, there is some near the top, it just doesn't seem to fully get rid of it all, but maybe thats because the motor isn't working so well?
3) There is a leak in the filter, this might be true since I had a bit of trouble last year with getting the head of the sand filter to stay on, instead of a bolt assembly like my old one, it had a cheap looking plastic ring clamp that would pop from the pressure last year. But my understanding is that this would more likely be a water leak then something that was accumulating air, since its after the pump.
Thoughts? Suggestions for next course of action? I suppose the right thing to do is to pressure test the piping from the skimmer to the pump, but I've never done one before.