I have searched the forums and haven't seen this exact situation. If it has been covered before please forgive me. I have an 18' by 36' inground pool with a 9' deep end; Hayward Superpump; 1 skimmer line; 1 main drain line. Looks like inch and one-half pvc pipe.
So, here is my problem: with both the main drain line and the skimmer line open(each has an independent pvc valve) there is no visible air leak. With just the main drain line open there is no visible air leak. When i close the main drain more than half way I get a sudden rush of air into the
pump basket. There is so much air that within a few seconds the pump basket is half air and the pump is barely able to maintain prime. Otherwise there doesn't seem to be a problem. I am seeing no noticeable loss of water anywhere. I have not found a leak above ground yet but I am hoping that is the problem and I don't have to dig up the concrete to get to the leak. It doesn't seem to me that it could pull that much air from an underground leak, but I am not an expert. I thought at first that I may have an obstruction in the line but water is flowing through it. I tried using water pressure to clear the line...it flows through o.k.. Now I am wondering if I have a partially collapsed line instead and it is starving the pump.
Thanks for taking the time to read this. Any input would be appreciated.