In my experience, water pumps on bikes and cars do often have a habit of leaking, then not leaking intermittently for a while before they sh!t themselves completely. I take it coolant is leaking (or rather was) leaking out of the drain hole on the underside of the housing? It's function is act as a tell-tale (and to prevent any leaking coolant from the mechanical seal getting into the oil).

The mechanical seal will need replacing, but you'd be sensible to replace the oil seal, gasket and the main 'O'-ring too. The bearing will probably be OK as will the two 'O'-rings that go in the external pipework, but obviously inspect them when you take it apart.

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