Hi - Recently (about 2 months ago) we changed our pressure tank and all was great. Came home yesterday and flushed the toilet and it sounded like it was going to explode. There was a lot of air in the line. I went and checked pump house and all looked fine. The tank, though sounded really hollow. I turned the closest faucet on and watched - the water had a lot of air in it and was rusty - but then cleared and was the pressure was fine. Had great, normal water all evening - but in morning same problem. When we use the water it is fine - but when it sits without use - air builds up. Please - any help would be awesome! A friend said the foot valve? What is that and where is that and if it's that - is it hard to fix? We have a Grundfos submersible pump from the 80's? I think. Thank you!