Bungee Plugs - Anyone ever used these?

So far so good. I guess I'll see how they hold up... I took a pic of it in the return after blowing the lines, but it didn't turn out.

Very little tension on the "bungee" Enough to hold it closed, but the cone shape with the o-rings molded into it hold it to its seat with the weight of the water.

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Ant update on how these held up?

I'm happy - as far as I can tell they held.

Just closed up today with them again. No issues - the rubber was still soft and in good condition. I smeared some lube tube on the o-rings and on the mating surfaces of the conical plugs. Pressurized air pushes them out just enough for bubbles to escape (maybe 1/16 to 1/8") As soon as the pressure stops the water pushes them closed. I went around and pushed on them out of curiosity and for good measure, but they were all seated.

Next year when I open I plan to put air to the system again first to see if any water pushes out, or if I get air immediately indicating if they remained sealed or not.

My only issue is when threading them in with a large channel lock I slipped an cracked one of my return face plates. Replacement was easy, though - swapped it out under water, and no issues with the liner trying to move or anything. Not sure if the cold water helped it stay in place, but in any case, easy-peasy.
 
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