The sand drain plug on my old Pentair Triton II filter started leaking. The previous owner had been using tapered water drain plug in the sand drain plug, rather than a straight-threaded plug with an o-ring. The portion of the sand drain plug that sticks out from the side of the filter had been cracked in half lengthwise. So I ordered replacement parts. Naturally, when I put a wrench on the sand drain plug, it ended up breaking completely off, roughly flush with the filter.
So I took my new sand plug to Home Depot and found a 7/16" by 2" or 3" bolt which fit nice and snug inside the sand drain plug... snug enough that I used a wrench to screw it into the plug. I then lightly coated the threads with Gorilla Glue and screwed it into the broken drain plug. Gorilla Glue foams and expands slightly as it cures, and water causes it to cure, so the Gorilla Glue would cure properly inside a damp sand-filled filter. I let it cure overnight and the broken plug came right out with a wrench on the bolt.
Photo shows both the broken and replacement plug.
Hopefully this helps someone else. I wasn't really looking forward to having to empty the sand out of my filter to get the plug out.