How can I eliminate air in my water lines?

A few mo.s ago I had my house water tank replaced. A few days later I noticed a very small leak from one of the solder joints. The leak stopped a wk later. I am still getting air in the lines. For over 30 yrs I never had air in the lines, so it has to be coming from one of the solder joints. Can I wrap the joints with teflon tape the cover the tape with say pvc compound or pipe sealer or something else? I told the co. about it. But I NEVER let someone give me an inferior job twice.