Underground Suction Leak?

[Knighaw]

Have had no issue with pool until yesterday and I am stumped..... I believe I have a leak between the skimmer and the pump. (Checked unions of PVC, skimmer gasket, skimmer top, and draing plygs)

There is just one underground(under concrete) line but the funny this is I only lose water while the pump is off. Also the system is sucking air somewhere and i cant find any where in the above ground plumbing, There are no drips or leaks or any thing of that nature.
Also, When i try to vacuum, within a couple of feet it the suction cleaner stops and The pump immediately fills with air losing the prime (Happens every time on either filter and waste).

The odd thing though is if I turn the valve from the skimmer to the pump to 3/4" open the pump primes. But as soon as i connect the suction cleaner it loses prime again.
My pool is very old so the main drain at the bottom was closed off 10+ years ago so it only has the skimmer with a line to the pump and one return line...

Any help would be appreciate. Also if i do have a leak under the concrete is there any test to be sure or as there any fix without digging up the concrete?

 

[Texas Splash]

Well, it's true that it may be a skimmer line or right under the skimmer. But I wouldn't place too much validity on the vacuum portion since vacuum lines always seem to let air seep in. From day one as a new pool mine did as well. But there's another possibility..... pool light? Do you have one? It could be leaking inside/behind the in the niche. If you has a plug to stop-up the skimmer intake overnight with pump off, you could confirm if the leak is solely within the pool or indeed somewhere in the line. Keep in mind it could also be leaking around the outer edge of the skimmer opening.

If the leak is found to be under the concrete at the bottom of the skimmer or the line going to it underground, it's just a nightmare of manual digging really. No other way around it. Some will "carefully" attempt to dig under, but you have to watch for the weight on the concrete above you. Others will simply cut that part of concrete out and dig down from above and fix it all later. If you rule-out the pool shell and light fixture, you might consider a leak detection company. With their high-tech devices, they may surprise you. But if they confirm the worst, they may offer a decent repair plan as well.