Leak Or Evaporation?

[JRRandall]

In the summer of 2010 I had my 28,000 gallon free-form gunite pool renovated (plaster, tile, coping, and all new equipment) to the tune of tens of thousands of dollars. That summer and last year we have been extremely happy with the new looking pool! I have been a pool owner for 6 years now and I have been opening and closing it myself without issue... until this year. This year when I took the cover off I was horrified to find that more than 75% of the water was missing. The water level dropped below all fixtures and outlets, and since I just had it remodeled I don't believe it could be structural either. The only other possibilities are evaporation (wishful thinking?), hydrostat pressure valve got opened somehow, or a crack in the main drain line caused by freeze/thaw (probably).

I immediately started to fill it up and soon realized that if it is a leak I am just waiting time and money on my water bill. I put a few thousand gallons in it anyway just to see if the water level would hold. I marked the sides with duct tape at the water level, and I grabbed a bucket to try the "bucket test" for evaporation. It has been 36 hours and much to my surprise the water level in the pool appears to be holding. I don't really know where to go from here... should I keep filling it up from the garden hose in small increments and check for leaks (ouch I have to pay sewerage fees on my tap water too), or should I hire a water truck to fill it up (cheaper, but only if there is not a leak). We had a very mild winter this year, and I have a mesh elephant cover so is it even feasible that the water did evaporate (I remember 2+ weeks in March of 85 degree weather)? When I close the pool I do drain it down about halfway (just below the diving board light) because I anticipate lots of snow and precipitation to help fill it back up (which if you live in the northeast you know that didn't happen this year). Should I hire someone to do a pressure check on the lines and potentially diagnose a leak? I really don't have much cash on hand right now so I don't really know what to do... Does anybody have any experience in main drain line replacement and excavation? I know it has to be expensive but I'm wondering approximately how many $$$$ am I going to have to part ways with? I don't think my pool would do well without a main drain for circulation. This is very discouraging. Any help/advise is greatly appreciated!

[JRRandall]

So the pool builder who did my renovations in 2010 seems to think it's definitely not evaporation and more than likely it's the hydrostatic valve... how in the world could that have been tripped when I'm sure the water level in the pool was much higher than ground water level. They did replace the valve when they did the renovations. I am beginning to think I didn't properly blow out the pipes and I have a very expensive problem on my hands. I'll cross my fingers and hope it's the valve though.