Sub pump problems

1st time poster.....many time reader....
I am currently having problems with my well. I share a well with 7 other properties that hardly irrigate and our volume of water used is relatively low compared to other private water companies in the area. We have a holding tank that is approx. 2000 gallons +. My problem is frequent cycling of the pump. Our switch is set at the 45 to 75 pound range and it stays within that range, but when properly charged with air it takes around 20 minutes to cycle even during heavy use. After a few days of nomal use it began cycling rather quickly about every 1 to 2 minutes between cycles. It appears the problem is rapid loss of air in the holding tank. I have checked the tank numerous times for air leaks without any success. We have had the tank pressure tested with no negative results or loss of pressure. Where is this air going and why are we losing it..????????? My take on it is if the tank it airtight then the volume of air should never change therfore the pressure should remain constant?.?.?.right??????

pasqually said:

... if the tank is airtight then the volume of air should never change therfore the pressure should remain constant?.?.?.right??????

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That should be the case with a bladder tank, but the air in a regular tank is ultimately absorbed by the water in the tank and must be either regularly or continually replaced in order for that tank to have a consistent volume of air within it.