losing prime when pump set to low

[abfab]

I have a 1 hp Hayward Super Pump (sp1607z2msc7). When I set it to low I eventually lose prime. As a result my Aquarite swg stops generating and gives check salt/cell warning lights. I have tried with both main/skimmer open and individually (I have a separate valve for each). Is this normal? I don't have a timer. I'm a fairly new pool owner so I ran it on high last year not knowing what the switch was for.
The o-rings on the supply/return lines were replaced in the fall. Is there one on the pump that could be causing insufficient pressure? I don't think I have a leak as my pool water level is stable. When the pump is on high it works fine.

 

[zea3]

You may have an air leak on the suction side. Check all the above ground plumbing joints for leaks by drizzling water over the joints while the pump is running. Watch for water to get sucked into the joints. Also make sure the drain plug on the pump is snug. You should also check the o-ring on the pump strainer basket lid and lube it with silicon pool lube.

 

[JasonLion]

No, that is not normal. It indicates that you have a slow air leak somewhere on the suction side of the plumbing. A fast leak would cause the same behavior on high speed. But a slow leak can be such that high speed is able to clear the air and low speed is not. A slow suction side leak would not usually be noticeable if you were not using low speed. You lose very little water, and there really aren't any other obvious symptoms.

There is an article in Pool School on locating suction side air leaks in the Solving Problems section (at the bottom).

 

[abfab]

Thanks, I will follow those steps.
Is any amount of air in the pump ok? On high I see a few extremely small bubbles (literally only 3-4). On low, there are now one or two large ones on the top (about 1 inch wide). Previously there was one massive one. I tightened the drain plugs and added silicone to the o-ring which seems to have made the bubbles much smaller. I didn't realize the lid o-ring could be replaced! I noticed an improvement when I replaced the other ones.