How to Find the Hidden Leak When All Else Fails

[poolside10]

After 2 divers, a bunch of dye, putty around the light can and skimmer,and a load of money, etc, my pool was STILL leaking.
As a last ditch effort, I checked the backwash valve and found it was the culprit. HERE'S where the surprise comes in! Even with the pump OFF the pool was losing about a half-inch a day. To boot, the pump and equipment are located ABOVE the pool level. Although it sounds completely impossible, the pool lines seem to have held a suction and sucked water uphill and through a faulty backflush valve. The pressure gauge on the sand filter always shows about 5 lbs, which is apparently still trying to push water through to the the sewer line. I don't care how crazy you think this sounds --- give it a try before you call the diver in!! I spent about $2000 and a lot of sleepless nights trying to find a 200 gallon-a-day leak, only to find out it was something as simple as this. After googling everything about pool leaks, I still don't find anybody telling you to try this. Even if the valve appears to be in good condition, I would at least replace the o-rings and grease them good. In my case, the entire top o-ring was missing along with part of the plastic valve. Sounds goofy, but at a total cost of about 10 to 15 bucks if you do the work, you're liable to find the problem (or at least eliminate one of the cheaper things to fix before getting into big bucks and headaches).

[noob]

My sand filter has a multiway valve with a sightglass on the side. When I backwash or rinse I can see the water flowing through it and when the pool is unning normally the sightglass is empty.

I have taken it off to clean before and no water came out. That could be an easy way to test. If no water comes out then the valve cannot be the problem.

[poolside]

Not quite sure what to tell you except that I finally got rid of one heck of a headache when I fixed the backflush valve. I also saved a boat-load of money by not having the repair people out again. My sightglass didn't show any water flowing through it with the pump running or not, but apparently water was running through and heading down the sewer. Don't know what else to tell you except this ended up being the final fix for me. It's the cheapest part of trying to find a leak, as I'm sure you know when the diver comes out he charges $750 for a one-time check and $145 an hour afterwords (in the Northwest where I live at least). Multiway valves have a different gasket, but I would certainly start there! If you google POOL LEAKS I don't think you'll find anyone starting at the backflush valve ---- and they're still pulling their hair out!