I have a severe leak when the pump is running, just started quite suddenly. We are losing about 500 gallons per hour when the system is running. I am certain it is not the liner, as the leak only happens during pump time. Anyway, my wife when to the local pool store and asked them. They said that it sounds like the sand filter is bad. They say this due to amount of water loss coupled with the fact that we have noticed a fair amount of sand/silt in the pool shortly after cleaning it (manually). When I say a fair amount, the actual amount probably isn't that great, but it is very noticeable in the deep end and areas when it can accumulate. Where the main drain exits into our yard (we are on a large pc of property) the water is exiting there where it normally would when I drain water during the winter months due to large amounts of rain here in the PNW. So, given all of that, does this seem like the likely cause or something else? The pool has been in operation for 20-25 years now, so it seems hard to believe it would suddenly have sprung a leak in the underground plumbing. I first thought it might be the valve itself as I have replaced the spider gasket a couple of years ago after I noticed more sand/silt that I thought I should. This fixed it, but there was no noticeable water loss at that time. Many thanks for any assistance offered!