Some background: I bought a house a couple months ago with a pool, which made me a first time pool owner. I've been lurking the site for several months now, and started off with the BBB method (in fact, it's all I've ever known ). Pool chemistry is exactly where I want it.
Anyway, now I'm starting to get more familiar with the plumbing; finding out where pipes go/what valves do as nothing is labeled. I've determined that since I've owned the pool, the main drain has been shut-off at the equipment pad. So inflow is split 50/50 between the skimmer and the vacuum line, which has had a Hayward Navigator (which I had to fix) attached to it full-time. I verified the main drain is off by diving down to it and covering it up with my hands... nothing.
So, at the equipment pad, I turned the valve to an even split between skimmer and main drain, and turned the vacuum line off. When I did this, the pump lost prime for a few seconds, and I saw a little air in the leaf canister, but it seemed recover and get prime shortly after.
Wanting to check my work, I dove down to the main drain again, and covered it with my hands. I felt very light suction, but not really what I was expecting. Looking through the drain cover, I saw what looks like a drain plug threaded into the main pipe opening. I didn't have time to get the scuba gear to remove the cover and get a good look at it yet, but obviously there was some water getting through it.
My question is, should this plug be there? If so, what is the purpose? If not, should I remove it?
Also one final note, I noticed what looked like the same plug rolling around in the skimmer (so not inserted) with two holes drilled in it. Any ideas on what was going on here? Advice on what I should do?
Thanks in advance for the help!