new pool owner, pool losing water, please help!!

We bot a home in mid april that has an in-ground pool. the pool is 20' x 40' and runs from ~3' deep to ~8' deep. It has (I believe) a solid piece fiber glass liner.

The filter is a big Hayward DE3600, the pump is also hayward. The filter assembly is equipped with a Vari-Flo valve and a chlorinator.

There are two inlets, one from the skimmer and one from the master drain. There is one return that T's somewhere underground and connects to two jets. Approx 25' from the pump house, there is a "Main Drain Valve". It is about 2' underground and enclosed in 3in pipe. There is a lid on the pipe, but it has a hole in it and there is some water and debris in the area of the valve".

Upon firing up the pump for the first time and letting it run, the water level decreased noticeably. I inspected the pump area because everything was wet and appeared to be leaking. I took things apart one at a time:

Vari-Flo Valve assembly:
"spider" gasket and lid gasket replaced and lubricated
Lid replaced

Filter:
Drain plug replaced

Chlorinator:
Drain bolt replaced

The pump runs like a Ferrari and the pump house is now bone dry. I thought I had solved the problem and was pretty proud of myself. I let it run overnight.

The next morning, the water level was about 1-2' lower.

I refilled, tried closing the Main Drain Valve, and ran it again.

The next morning, the water level was about 1-2' lower.

I spoke to a pool company and the guy said that there was definitely a leak in one of the lines. A pressure test would identify the leaking line and options could be explored to repair it. The Main Drain was NOT going to be tested because this requires a diver. However, the guy assured me that if the other 3 lines held pressure, it would mean that the main drain was faulty. He charged $300 for this service.

I was not able to be home when they came but my wife was (it was a crew, not the guy i spoke to). They tested the skimmer and the returns and all three held pressure (my wife witnessed the tests) The guy said that the Main Drain was almost definitely leaking, but that repairing it would not be cost effective. He said to just run the pool on the skimmer, and that we may have to vacuum more often.

I was happy with this result.

I came home and closed the intake valve coming from the main drain and ran the skimmer. Several hours later it was clear that ~2in of water had been lost. The pool guy said that "it has to be the main drain".

I find this hard to believe because when the filter is not running the water level remains constant. When the filter is running, the Main Drain inlet is closed AND as i said before i also closed the main drain valve.

Does anyone have any ideas on what to try????

Leak detection is hard, in my experience.

Or rather, it's hard to find "THE" leak. It's usually not too hard to find "a" leak. Unfortunately, while I'm not great at leak detection on site, I'm really terrible at leak detection via the Internet. We can wish you the best, but that's about it.

Good luck!

There are a couple of "stickies" at the top of this section of the forum about finding leaks. Don't know if they would help you or not, but it couldn't hurt to read them and see!

Wow.....1 inch equates to about 500 gallons in your 20x40 foot pool. 2" loss in a few hours is a lot of water going somewhere. Very strange that you get NO loss when the pump is off but if that is really the case then something is opening up under pressure and is virtually closed when the pump is off. If the plumbing is PVC the only way I can imagine getting a leak that behaves like that would be a very long crack lengthwise in a pipe. If the pressure test was done right it certainly would show ANY crack. The leak would also have to be in the return side since if it was on the suction side you would suck air. When I pressure test my pipes I blow the water out and cap the returns. I then break the joint after the filter and hook a pressure test jig to the return pipe and pressurize to around 10 psi and it should hold for hours. The only reason I don't go higher than 10 psi is I don't want to blow the lid off my chlorinator.
I find it hard to believe there is no loss with the pump off. May be very small. You can try turning off the pump and letting the water settle and get still for a while then squirt some dye into the returns and see if it gets sucked into the return pipes. Try capping one and squirting dye in the other and visa-versa. Just for completeness squirt dye in the skimmer too. A big assumption on my part with the dye is the leak is below the pool water level.
Al