I would check to make sure that all the clips are tight that hold the radiator hoses on, fill up and test it again. I had a leak in my raditor hose once and have still yet to find out why the water level keeps ever so slightly going down in the bottle over a few months riding.

If not and it is probably the cap, it could just be a worn rubber seal, a new radiator cap shouldnt be too expensive.

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Plough on