I have a shower tap that keeps leaking. I have reseated it 3 times with a tool that grinds down the metal to remove a groove that that reappears in the same spot. I began to think the brass is stressed in this spot from the casting cooling too quickly, and thus corrodes along this line again and again. It takes about a year. As it is in a tiled brick wall, it would be a lot of work to replace it.
You could be right, or it could be a natural wear site, or, if a mains water pressure shower and the groove goes radially across a sealing face, it could be cavitation erosion/corrosion at an initially small leak path. There is probably not an easy fix other than to do what you are doing, or replace it altogether. It's presumably just part of the body casting, not a replaceable part. If you know the make, someone might have a suggestion.
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