I have a 16'x32' free form shaped inground and recently have had alot of water getting behind liner and i mean alot. in a 48hr period it will take about an hour or more to pump out the water from behind the liner using a small pony pump. the liner is only about 4 years old and i've scoured every inch of it looking for pin holes or anything out of the ordinary. i've tried using die around all return lines, skimmer, main drain, stairs, and seams. nothing. the water level in the pool does not change so the water doesn't seem to be getting away, at least not too quickly. i'm at the point where i need a professional to come in but i seem to be having trouble getting anyone to come out. i've read that there might be a malfunctioning hydrostatic valve in the main drain that might be causing the problem. not sure how to check or repair this valve. does anyone have experience with this or another possible theory?