I don't understand how it can start leaking, though. Going back through my records it was October 2015 when I fitted a new fill valve, and it's been leak free until a few days ago. Does the fibre washer perish in some way? The old one looks pretty ropey, I must say. It almost fell apart.
MM
It sounds a poor quality. I would expect it to last longer.
I can't really think of a plausible explanation for the failure.
I've only seen them fail when the two fittings are not perfectly aligned so that, immediately or over time, there was unequal pressure on opposite sides of the washer, and the washer was squeezed out at one point.
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