No suction from main drain & one skimmer

[RecurveArcher]

We purchased our house this winter, came with a ~40 y/o pool that has a single main drain, 3 skimmers, 3 return jets, and what appears to be an old vacuum port that's been sealed shut; see below for the basic layout. The equipment pad has 4 lines coming in, labeled Drain, East Skim, Vac, & North Skim. There are two lines returning to the pool, one feeds both jets on the south wall and the other feeds the return on the east wall (used to also feed a slide that we removed, which was directly above the jet). All skimmers are just below the pool coping and the jets are maybe a foot down.

At first I thought maybe they just labeled one of the skimmers as 'Vac' for some reason, but the line had the valve in the off position and the handle removed; I later found a port that appears to have been sealed shut in the pool, so assuming that used to be the vacuum port. The 'East Skim' does go to the skimmer on the east wall and the 'North Skim' pulls from the skimmer in the deep end to the left of the diving board. There is no apparent suction in the skimmer at the north/shallow end; I don't see anything blocking the line, tried to feed in a hose, but it stops a couple feet down where it takes a right turn. I also don't appear to be getting any suction out of the main drain; in fact, if I turn the valve on there appears to be an air leak as there's instantly bubbles in the pump, not enough to cause it to lose prime, but definitely full of bubbles that come out of the returns as well. If I shut off the main drain, the pump window is perfectly clear. Swimming down to the drain, I can feel something in the drain line, but can't tell if it's just clogged shut or if it's been intentionally sealed.

I've seen a number of posts here saying that the main drain isn't required, and some even saying they prefer not to have it. Due to the fact that it's either clogged or intentionally sealed, plus appears to have an air leak, should I even try to get it open or just leave it off?

Any idea which line the north/shallow-end skimmer would be tied in to? Seemed odd to me that everything else has a dedicated line but that one; not sure if it could be tied to the main drain and not getting any suction because of the blockage there? And again, how much effort should go into clearing it, or is the pool okay to run on just the two skimmers?

Also, the two working skimmers don't seem to pull very hard; the one closest to the diving board (also closest to the pool equipment) pulls in some debris, the other maybe 40% as much, but neither really as much as I'd expect. The returns are reasonably strong, so I know it's pulling water.

Basically, it's a pretty big pool, are 2 skimmers sufficient to circulate water properly?

 

[Texas Splash]

A pool that old and large could have various plumbing scenarios. It's possible the one closed skimmer and main drain were on the same plumbing line which is why they are both not working right now. I wouldn't worry about the MD. The skimmer(s) will provide the biggest water clearing benefits along with vacuuming or an automated robot-type device if you have one. It's also not uncommon for skimmers to share a line with each other and control flow inside the bottom of the skimmer with a diverter. Normally two skimmers would be sufficient. It's a shame the north (top) one is blocked because it would be nice to have surface cleaning capability at that end of the pool. Right now most of your circulation is on the deep end. Still not the worse thing, you may just need to monitor circulation a little more on that (shallow) end. Maybe on day you can re-plumb it to the pad and get it going again.