I've been to this pool 4 times. Its an 18x36 inground with a vinyl liner. Skimmer, main drain, and one return. Its losing 3"-4"/24 hr. The water level is currently below the skimmer face plate, no leaks found with the leaktrac, no suction of dye around the stairs, return face plates, or the main drain. Plumbing pressure tested ok all around. I can't see/imagine where all the water could be going. The liner is pretty old and one corner separated from the bead. Any Ideas???

We all have these kind of jobs that we have to go back to. Some on this board do leak detection as a hobby and can afford these multiple trips but others who have to make a profit, payroll, taxes, etc. would quickly go out of business if they couldn't locate and repair a leak right away.
Some of us have reached out to our competitors and can call and ask for another brain or leaktrac on the job and we reciprocate when asked. After all, it's all about the customer and solving their problem.

It might sound simple, but more than a few times when a pool is losing that much, and you cant find the leak, the backwash valve is failing and losing water. see if the backwash pipe is run somewhere where you cant see, sometimes homeowners do that so they dont have to roll/unroll the plastic flex pipe.
Ive found this to be the problem on a few pools....gl

Check all the pipe fittings, sometimes the fitting is cracked, or a gasket leaking and the pressure test plug covers it or goes past it, the leak can be in the first inch of pipe.
Also when all else fails: look for stupid things like somebody who takes the front plug out of a skimmer basin when that port goes to the ground.
I had one this week with a crack in the stair tread.
3 - 4" / day, might try a hydrophone, that big a leak can be heard
Please post the answer when you find it!
Thanks
Bruce

3-4 inches in 24 hrs.....would take about 15 mins to find in that pool. isolate the plumbing with plugs....does the level drop?. does the level continue to drop below the skimmer face plate....if not it is in the skimmer basin...is there a lite?.

3-4 inches a day could be multiple small holes(still very obvious) or 1 large hole very obvious.
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I agree with bruce....that big could be heard...I think it could be seen!!!

Check the backwash!!

If all obvious culprits are sound, think outside the box. Here in Los Angeles we don't run in to too many vinyl liners, but I've done enough to offer a couple tricks. Dye test the threads of any eye-ball fittings your plugs may be sealing during pressure test. When you are diving it, do a grid pattern search along the whole pool. Look for algae along a seam, any rough spots in the bottom by rubbing your hand back and forth, or debris in a certain spot. I have found a lot of pebble-tec leaks by slowly swimming and seeing dog hair, leaves etcc.. sucked up into a hair line crack.

Subcontract out to a company that knows what they are doing!!!

lol 4leakman

My teacher has left my company. I donated the floor of a fountain, but when I cut the leak was not there. I am now donating again, but hear nothing. Leak is in md lines (2). Maybe 20' before it hits equipment. Any advice?

My advice is to start a new thread and explain yourself better. What do the terms "donate" and "overlay" mean in terms of leak detection?