What a mess. This occured some time ago but felt it was worth while mentioning to all you leak detectors out there. Got a call from the pool company who just replaced a liner, rehabed some parts of the pool to include the main drain. Pool company stated the customer had a very large amount of water loss during a 48 hour period. They did not know leak was coming from. I dove the pool, as most of you would probably consider and went for the main drain. Opened up the cover and found the hydrostatic valve wide open. Hayward makes this hydrostatic value that looks like a see saw, but its not joined to the main drain body in the middle, however on one end. Its not a threaded valve and actually rests in a snap in cradle. The valve was positioned straight up, and did not close. Anyway, just wanted to mention that the valve was warped on the end that joined to the drain preventing its falling down into the sealed position. Bad design by hayward. But they do make a nice replacement part number sp1056 and will accomodate both 1 1/2 and 2 inch abs. This is the first main drain i have seen with this type of hydrostatic valve design and its not a good one.