Can't find my leak

[scomack]

my set-up:
vinyl lined in-ground pool but with a complete concrete shell.
2 skimmers and main drain on bottom all on south end of pool.
Pool pump around 30 feet due west of the western-most skimmer.
3 returns on the norther end of the pool in the walls.

symptoms:
I lost about 6 inches in the last 48 hours with pump NOT running.
I lose it at a faster rate (possibly twice that rate) when the pump is running.
I get some bubbles in the first return when the pump is running.
I've lost 1.5 feet (or so) in 5 days.

Since the pool is concrete lined, a vinyl leak would be contained, would it not? I see no bubbling behind the liner (plus level wouldn't fall.)
I see no noticeable wet spots on the pool-to-pump pipe route.
The route from pump to pool goes under quite a bit of concrete .
The exact routing of the plumbing is not known.

What should my next step be?

 

[duraleigh]

Why not let it leak down until it gets below all the liner penetrations to see if it stops?

 

[scomack]

Why not let it leak down until it gets below all the liner penetrations to see if it stops?

I'm getting close to that point. It is right at inlet mid-point level and the top of the light. That is the last of the penetrations.

 

[scomack]

The leak has slowed but it is still falling. It is below the inputs now and at the halfway mark of the light.
Tomorrow should let me know if it falls below the light housing.

 

[scomack]

The water level seems to have stabilized. It is about 60% of the way down the pool light. The only other penetration of the liner are the stairs. They are a hard plastic "pre-fabbed" stairs that are inset into (outside) the perimeter of the pool. Roughly 1 step is still submerged and the other 3 are above the water level. I felt around each item for softness behind the liner and felt none. I've inspected the liner from above and can't see any problems.

Any thoughts or advice on what else to look for/check/test?

 

[Bama Rambler]

I'd suspect the light niche if that's where it stopped.

 

[scomack]

I'd suspect the light niche if that's where it stopped.

So what would you recommend? Getting a new niche gasket and applying that? Tightening the niche-to-wall screws? Trying to add water and do a dye test? Would you suspect leaking behind the niche and going behind the liner? Going through the niche and into the ground behind the wall? Following the electrical conduit?
My liner isn't bulging, so I would (in my guestimation) eliminate leaking behind it. I wouldn't suspect leaking down the conduit to the point of electrical connection because the level fell too quickly for that small area and I saw no water around the connection box which is at a higher level anyway. There is no gasket that seals the light enclosure into the niche area, so there is nothing to fix there unless the conduit is broken and water is leaking out of it.

 

[Bama Rambler]

It's either the niche body or the conduit. You can remove the light and try using food coloring to find it.

If it's the conduit you can seal it up but that's not the right thing to do.
If it's the niche itself you will have to replace it.