My pump and filter are about 20 ft and slightly down-hill from my in-ground pool. I have a new SWG and Pentair single speed pump. The plumbing includes a skimmer and vacuum line, and a single return. The water level in the pool is approximately 24 inches higher than the pump. If I don't shut of the valves to the two vacuum lines, and remove the pump's basket cap, water floods into the pump - as you would expect. So one might think that as long as the pool water is adequate there should never be a reason for the pump to be dry.
However, the pump basket is often completely dry after having been off has been off for an hour or so. The presence of the air in the pump basket is preventing pool water from entering, and the pump seems to have difficulty clearing the air out of the line. When I see this situation I will normally shut off the pump and briefly loosen the pump's cap which allow the gravity-fed pool water to rapidly fill the basket, and I then re-seal the cap and start the pump.
There is no evidence of any water leaking, and since the pool is above the pump, it would seem that if there were any kind of leak in the vicinity of the pump or filter, that gravity would keep the pump basket from becoming drained.
MY CONCULSION: I am speculating that when the pump is off, dissolved air & gases are collecting and creating enough pressure to force the water out of the pump basket. I am also speculating that if I allow the pump to continue running for a minute or so the pump will be able to clear the line, but I am a bit concerned about running the pump when it is dry.
Anyone have experience with this type of a phenomena/symptoms when the pump is below the pool level??