hi everyone,my pool intermittenly loses water,sometimes a foot over night,sometimes not at all.for instance,this winter the water level stayed the same all winter long ,i started to fill it and it seemed fine until it reaches a certain threshhold then empties ,sometimes 1 foot overnight .is it a valve near the drain that opens only when the water pressure gets high enough?do i have to drain the pool to check the drain?what about the liner,will it sag ,ect.....thanks for any input,johnny