Mysterious pool problem

This one has me stumped. I have an inground, vinyl liner pool. It has a main drain, 2 skimmers, and 2 returns. The sand filter has a port for skimmer/main drain lines on the suction side.
When I opened the pool, the water level was quite low. Starting up as I normally do (after filling), I have great suction on the skimmers (with full port) and no suction on the main drain (with full port). It also loses about 1/2" water daily. So I assume it is a leak in the main drain. I plug the main drain in the pool, and the leaking stops.
But....with the main drain plugged and the port on full skimmer, I no longer get suction on the skimmers. Nothing. If I unplug the main drain, I immediately get full skimmer suction, good flow, no bubbles, perfect. I now re-plug the drain and within 30 seconds the skimmer suction no longer works, even with the main drain plugged and the port on full skimmer.
What gives? What does the main drain being plugged have to do with the skimmer suction? TIA, Bob
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What's happening is, the main drain "input" isn't fully closing when you choose skimmers only on the multi-port selector. Since the main drain is plugged, you're eventually sucking air from the main drain line instead of water, which is causing you to slowly lose total suction power.
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This makes sense - would you recommend I simply cut out the multi-port and direct connect to the pump, until I eventually decide to repair/unplug the main drain? I guess that's the only solution if I want to filter my pool..... Thanks, Bob

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I would physically remove the main drain input and, if the multi-port allows, screw in a threaded plug.

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That did it, exactly as you said - thanks a big bunch for a quick and right-on diagnosis! Bob

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You're welcome. Feel free to send a PayPal payment to my e-mail address :-)
All kidding asisd, I wonder why it is so many people have problems with these main drains?

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Sounds like the valve gasket is gone over a couple of the ports. Happens to me about once per season. RJ

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