Closing the valves to the skimmers and drain has isolated the problem to the PUMP side of those valves. Look at it this way - your pump/filter is the high point in a continuous circuit from pool to pump to pool. If there is no leak, any tendency for water to run back into the skimmers and drain is countered by the pulling of water from the returns up to the pump. Just a like a siphon or drinking straw. If there is a leak near the pump or filter, gravity will tend to pull the water back into the pool through the skimmers and drain, and through the returns in the other direction. The water is replaced by air leaking in to the system. By closing the valves, you prevent the water from returning to the the skimmers and drain. Odds are, the leak is somewhere in the equipment at the pad. To isolate between pump and filter you have a few options. If it is a very fast leak, you may be able to listen for it when you open the skimmer valve with the pump turned off. If you have unions between the pump and filter, open that line and plug (or tape) off the pump side, open the skimmer valve, and see if you lose the water in the pump. If so, the leak is between the union and the skimmer valve. I have clear pipe between my pump and filter, so in my case if air was coming in at the filter I would see air in that line moving toward the pump. Also, if the leak is at the filter or anywhere on the pressure side of the pump, you would expect to see some water coming out when the pump is on UNLESS it is leaking to waste from your multiport valve. Do you have any way to see what's going on in your waste line (a union to disconnect it, or a clear section, or possibly watching what comes out the other end)?Originally Posted by h2o
Adding a check valve may very well solve the problem by preventing the effect of the leak, but it won't fix the leak.