Hydrostatic Relief Valves can be located along the bottom of the main drain pot in the pool. They have a spring loaded mechanization that lets water in automatically when the pool is drained. Also known as a Hydro-valve it prevents ground-water pressure from raising the pool or causing movement. The hydrostatic valve prevents this by allowing ground-water from your surrounding yard to flow into the pool and relieve the pressure on the pool structure. Anytime the pool is emptied the hydrostatic valve needs replacing. Once the spring in the hydrostatic valve opens they become unreliable and can cause the pool to leak.