I did a pressure test on a pool recently to try and narrow down the source of some vacuum problems. The issue is that after about 5 minutes, the pump starts pulling a ton of air out of absolutely nowhere. From then on, lots of air moves through with the water, making vacuuming very difficult as the pump goes through a repeating prime-run-cavitate-try-again cycle. This ONLY happens when vacuuming. A new hose did not help. This problem has gotten a little worse every year.
First, let's verify that I did the test right. This particular 25-year-old pool has dual main drains and two skimmers. The two skimmers come into the pump house in a single line; they are not separate. I plugged one skimmer with a threaded plug and closed the skimmer isolation valve near the pump. In the other skimmer, I set up an assembly consisting of a pipe with a threaded adapter on one end (screwed into the skimmer), and a pressure gauge, a ball valve, and a hose bib at the other end.
I tested the incoming water at 35 PSI. After hooking up the hose and turning it on, I opened the ball valve. I expected to see the pressure climb to the 30-35 PSI range, same as the hose pressure, at which point I would close the ball valve and see if the pressure dropped over the course of 10 minutes or so. What I got instead was that the water pressure never climbed over 4 PSI, no matter how long I left it on. Perplexed, I traded the plug and moved to the other skimmer (closer to the pump house). At this skimmer I got 7 PSI, and again no higher. Just to be sure there wasn't a hidden cross connection to the main drain, I poured a bunch of dye into one skimmer and forced water into it. The dye came out of the other skimmer, but not from the main drain or any other location.
I believe this suggests that I have a large break somewhere in the skimmer line. My theory is that the ground around the break is flooded (from leaking), and after years probably has a hollow around it. I think that when I am vacuuming, three or four times the normal suction is put through one skimmer, pulling water back in the break - and after roughly 5 minutes of this, the hollow is evacuated and air starts getting pulled in.
Does this sound right, or am I way off base or missing something? (I will be doing a pressure test on another site on Monday so it would be good to know I was doing it right...)