Thanks @wireform , They're actually pretty small so I'm guessing the 3/4 openings. I did switch them and it didn't do anything.
I did notice that one of the 3 pool returns, the one the farthest away, doesn't have as much pressure as the others. I wonder if there is a return leak somewhere under the concrete? I also wonder if that could cause the pump issue... but everything I've read seems to suggest this is suction side. My buddy is a pool man and he came over this afternoon and said that he has several customers with the same pump that runs like mine does. Said it's no big deal.
After all the things you've tried, I am also inclined to say "leave it" and simply monitor. If the basket level doesn't drop any further than what you typically see, and prime is not an issue, leave it be and enjoy your pool before it drives you nuts. I'm incredibly OCD about my pool, but I've had to make myself step-back and just some things go.
Having said that ... ha ha .... if you were ever in such a position to get experimental, you could run a straight suction pipe to the pump - like a big straw without any valves, check valves, and using any size you wish to see if it makes any difference in water pull and basket fill. If it does, you know it's a suction-side issue of some kind. If it doesn't change anything, then you could attribute it to either the way that pump runs, or perhaps something on the pressure side. But if you are able to run on recirculate that would seem to get ruled-out quickly. Nothing that appears to be a "must-do" issue, but only if you have a few spare parts, time, and can't sleep thinking about it all.
Lol @Texas Splash , too late... think I've already driven myself crazy. My contractor suggested the "straw" test with a 2 inch line... but just seems like a pain. I seriously doubt that would make the difference, but I'll probably never know.
I'll monitor and make sure it's still running solid and hope for the best. And enjoy my pool.
I appreciate everyone's advice... definitely still open to more suggestions if anyone thinks of anything... but I'm ready to move on if nobody else has any other ideas. Probably will only lose a little sleep. But I'll get over it.
As Texas Splash asked "1 - Any chance you have an obstruction after the pump? ", you do in my opinion, if I look at your picture correctly, have a obstruction in your line, your heater right after the filter. You may not agree with me, but let me tell you my situation, even if I don't completely understand it, but this is the way it has always been on my pool.
New pool in 2014. water comes to pentair vs pump from 1.5' skimmer and 2' main drain from pool. from there to sta-rite(pentair) DE filter. after it leaves filter, there is a pentair valve that I have to set, either to return to pool, or to run through the heater, (the same way yours does), but you have no valve there to turn it to let it go to the pool without running through the heater (the obstruction). Like I said, you may disagree with this story, but it's the way my pool works since day 1.
OK, If I'm not heating water, water back to the pool, no air in pump basket filter under clear lid at any speed. From 1200 to 3000, no air. For hours during the day at 1500 to skim for good Florida pollen (lol), no air. Turn off pump, over night, it's a closed system, no water drop out of filter, pump. anywhere.
OK I turn valve to run water through the heater to heat the pool. Here's where it get interesting. I run the heater (mostly in spring or fall to keep temp up from cooling nights), at about 2000 to 2400 depending on air temp. It's a Pentair Ultra-Temp, not a gas unit. Just 400 rpm makes a little difference, and the throw of the valve a lot of difference The heater, the obstruction. At 2400, I get a very small amount of bubbles in the basket, at 2000 rpm I get a little more bubbles in the basket. They will keep accumulating a little more, the longer the water runs through the heater. OK, so we're swimming, and the pool heats up to temp set, the heater shuts down, I don't get out and turn the valve to stop the flow through the heater, and later in the day when out in pump shed (as I call it), the basket from running all day has even more of a bubble in it. Pump turns off later in the day automatically. The next morning I go out to the shed, and I purposely leave the valve thrown to run through the heater, without heating, so to just have the clorine running through it. My vs pump ramps up to 3000 on 1st start up, and then down to my pre-set speed of 1500 for skimming the pool. The 3000 will clear the bubble from the basket. But at 1500, through the heater, the bubble will start accumulating through out the day, and if left to run all day like that, will be a pretty good size bubble. Water above the basket, but sloshing around enough you can see it's movement with the pump running. I can go out and close the valve later in the day, to isolate the heater out of the flow, leave it at 1500, water to pump-filter- to pool, the bubble will be drawn out and disappear, even at 1500. No water through the obstruction after the filter on it's way back to pool--the heater.
You may find this strange, I do. But it's what my pool does, and has done from day 1. I don't think I have an air leak either, closed system, the water (with everything off), never goes out of the basket, or the de filter, (I can open the air blled valve at top of the filter), and get just water--no air.
It could be worth a shot to isolate your heater, to try it.
I agree with you 100%... I have a couple of houses with DE filters.. I no longer backwash and just clean the filter twice a year... I have notice that at my normal 1200 RPM, I have more bubbles under the pump lid when the filter is dirty, than I do when the filter is clean.. Even when dirty, I still get plenty of water flow and my pressure is usually 6 to 8 PSI. Like you, the difference is subtle and the water in the pump never drops below the top of the basket.
Thanks for your input and detailed post. You're right... doesn't really make sense... but not saying you're not right either. Actually, at first blush, seems to go against @JohnT 's theory of creating more resistance by turning the eyeballs. Hmmm, maybe I'll plumb in a bypass valve after the pump and before the heater if I have room.
I still tend to think it has something to do with my old filter, which has leaked before in the past, but nobody else seems to agree with that theory either.
Again, thanks for sharing your experience with us.
Also, that thought never crossed my mind because the heater is only a few months old and was added by me when the house was purchased... and I've always had this issue with the pump, though it's muuuuch better now (it used to be completely cavitated and blow bubbles in through the returns) that I've re-plumbed most of it and got rid of the big leak in the pool. The water does go over the top of the basket even on low rpms and swirls to part of the top of the plastic lid on higher rpms... but never completely to the top. Interesting theory though.
I've been following along and thought I would chime in... sounds like an obstruction after the pump to me.
-I had a similar situation and it wasn't until I pulled the impeller and saw it was clogged with little oak flowers from our oak trees did it fix the issue. I saw you checked your impeller.. but you might want to pull it out and take a closer look. I did a cursory check of mine and even bought new filter cartridges thinking that was the problem, but when I pulled it apart it was obvious.
-I used to have a DE filter and it did the same thing when it got dirty.. in fact that was a better indicator for me to clean it than the pressure gauge. I didn't see if you tried running without your filter grids.
-I don't agree with the pipe size argument either. I upgraded/moved my pool equipment similar to you.. and I put in 2 inch PVC... but the old stuff was all 1.5 inch.. neither arrangement caused a B-in-B (bubble in the basket)
-sounds like you are handy and can try some stuff.... have you tried plumbing a temp return line from the pump that just dumps into the pool? I curious if you get the same issue when there is no head pressure at all from your Pool equip.