I've searched and found countless threads on this topic. The general consensus is air in the pump basket is caused by a leak somewhere on the suction side between the pool and the pump, or by a leak in the lid gasket. However, I'm not so sure that's the case here.
So first a little background.
The pump is a Purex Triton WisperFlo 0.5 HP, with an AO smith Century Centurion motor. I have no idea how old it is, I just purchased the house last summer, and that's what it came with. The air in the basket has been there since we moved in. In fact, I didn't even realize it was a problem until I saw other threads here talking about it.
In my case, I see several 3" or larger bubbles under the lid (one bubble in each vane of the lid), and the bubbles are noticeably being agitated as the pump is running. I'm fairly sure it looks like the bubbles are coming from the impeller side of the pump.
I've checked for leaks in all above-ground pipes by using a spray bottle, as well as dumping a bucket of water on pipes and valves, as well as the pump drain plugs -- no effect.
Switching from pool mode to spa mode (both suction and return) has no effect - same level of air in the basket.
I watched the pump basket while having my wife restrict the flow both on the suction side, and on the return side. Again, it made no difference in the amount of air in the basket.
I've recently replaced the lid gasket, and it is lubed with plenty of silicon. The lid is on VERY TIGHT, and I'm positive the leak is not coming from the lid (especially after performing the water test).
My filter pressure is on the low side. Clear baseline is 10 PSI. I have a booster pump for the spa jets, and when I turn that on, my filter pressure jumps to 16 PSI. I don't really understand why that is. The previous owner had the baseline pressure on the gauge set to 10 PSI as well, so I know this was true when he was managing the pool as well.
My filter always has air in it. When I bleed the filter, I'm always able to bleed air for about 5 seconds before water starts spurting out. It takes about 30 minutes for the air to come back once it's been bled.
At this point, I'm thinking the pump may require dis-assembly to inspect farther. With the pump off, and the basket out, I don't feel anything obstructing the impeller, and I'm able to freely spin it with a screwdriver.
Can any of you folks with more experience than myself offer a suggested plan of attack? Am I correct to think the likely culprit is something with the impeller, or possibly diffuser o-ring? Should I have replacement parts on hand before I tackle this? I don't want to end up in a situation where I can't get my pump running for an extended period of time!