I have a 3/4 hp. Deep well pump out in the lake about 300 ft from shore . it has worked well for 5yrs, but the past yr I keep getting air in the lines .the line is good and nowhere is there pressure drop , I have also clean the pump and it seems to function well . It seems the longer the system is not used the more air I get . The pump is in 5 ft of water ,so its not a case of no water . Any ideas what could cause this ?

Are you sure this is a deep well pump? I would think you have one line running 300 feet into the lake and ending up in five feet of water. Is that the case?

If that's the case, I'd take a look at the foot valve which, it would seem, is at the end of 300' of pipe in the lake. If it is leaking slightly, then over time (and you stated time is a factor in this) it could leak down enough to allow a little air in the pipe. If that is the case, then I would think that running water for ten or fifteen minutes is enough to clear the air and you have no problems after that until the system is down for some length of time again.

You are pumping into a bladder-type pressure tank... correct? If not, then that changes things.

I hope no one is wading or swimming in that lake...