Stop in for a cup of coffee

Your tank sounds like it is water logged. Turn the pump off and let 5 gal or so of the water out. Then pump the tank up to 40 pounds with your air compressor. That will fix your constant running.
How will that help? These things have a bladder. Once it gets water in it, it's done. I let the water pressure down to 0 to check pressure on the bladder. I will do that again in a few days and see if it still has 18 PSI. If not, I suspect it has a small leak. If so, I wouldn't want to over pressure it or I could split it open and then It would be 100% pump run...