I don't think I ever posted this tutorial with pictures, but I've mentioned it a few times.

These plugs are useful for anytime you're working on your brakes and are really easy to make. If you're pulling your calipers you can install these in the ends of the soft lines to keep them from dripping. If you're replacing your master cylinder these will let you bleed the MC. Keep them handy.

You can use a similar process with the female fittings on an old rubber line to make female plugs.

Here we go!

First snip a section off of an old brake line, or a universal bubble-flare line from the parts store.
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Crimp the end shut, to keep the solder from running out when it's liquid as well as to keep the nut from sliding off the end when not in use.
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This is plenty of crimp.
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Cut a small piece of thick solder that will slip inside the tube.
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Slide the solder into the tube, and let it drop down to the crimped end.Click to view attachment

Using a pair of pliers or a vice or any other suitable device, hold the brake plug upright as pictured and heat sufficiently to melt the solder. In this case the crimp was glowing slightly red hot.
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Let cool or spritz with water. And make as many as you need.
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Good idea. But that isn't plumbers solder.

Why not just use flare fitting plugs?

Good thing I'm not a plumber or an electrical engineer.

I'm sure purpose built stuff exists, but most people have this stuff in their garage on-hand and it takes 5min to build. So instead of putting off a job to order stuff or trying to come up with a work around, they can just do this.

Well, I suppose that might be true, but just about every town has at least one plumbing store. I have 5 within a mile and I live in okiehoma

Because it's what you can make/build and not what you can buy......