This post is more for informative purposes, as maybe it will help someone in the future with a suction side leak.
I had a big suction side air leak a month ago and did practically everything that the regulars on this site recommend, including:
-Check water levels, weir door
-Lube pump lid O-ring
-Replace pump lid O-ring
-Replacement pump lid cover
-Lube 3-way valve O-rings
-Replace 3-way valve O-rings
-Replace pump gasket (per my pool guy)
-Plastic wrapped - shaving cream the above-ground plumbing (the plumbing before the 3-way valve)
-Replace above ground plumbing and 3 way valve
-Blow CO2 through the lines in hopes of clearing any possible blockage
Nothing I tried fixed the problem. My pump was an ancient single speed, and I was debating on replacing it with a new variable speed pump anyways, so my pool guy and I decided this could potentially fix the problem. When we swapped it out, it did help the pump stay primed, but there was still major air coming in.
It was at this point we decided to call out the leak detection squad (minimum charge of $195). Low and behold, they found an underground leak on one of the skimmers, about 5 feet away from the skimmer beneath the concrete. It took them about 15 minutes to find. I moved some of the surrounding pavers to create a path to the leak, and had my pool guy come in with a concrete saw to cut out an 18"x18" section of concrete. He found the culprit, a hairline crack in a 45 degree elbow. He made his repair, fired up the pump, and no more air! Poured new concrete, added texture, came back and painted the next day and put the pavers back in place. Cost was $450. A month after discovering the initial problem, my pump is finally working great. Unfortunately, I spent over $2,000 trying to fix the problem.
In doing all of my research on this website, I read many times that underground leaks were extremely rare and 99/100 the problem lies above the surface. This is probably true, but when I read this I was convinced it was impossible for me to have an underground leak.
My moral of the story on fixing suction side leaks: do all of the free/cheap repair possibilities first, like checking water levels, O-rings, shaving cream, etc. If the cheap fixes don't help, call out a Leak Detection company to test your lines. I should have done this before I went and replaced the 3-way valve, above ground plumbing, and a brand new pump!
Thank you to all contributors on this site, as I would not have known the potential fixes for this problem otherwise. I hope this post helps someone who is pulling their hair out over a suction side air leak.