I'm getting air in the incoming water to the pool pump. I have narrowed the problem to the piping coming from the main drains in the bottom of the deep end of the pool. When the valve to that pipe is closed, no air enters the pump. Therefore, I've eliminated the piping from each skimmer. When I open the valve on the main drain piping, air enters the pump. Also if you put your hand around the PVC, you can feel the vibration of air bubbles flowing through the PVC. Unless the piping underground that is 14 years old has developed a crack, my first thoughts is that the air is entering around the shutoff valve handle when in the open position. Is this the first logical place to look for the problem? From what I can see, it doesn't look like it is serviceable so it would need a complete replacement.