I think the theory is that the heat causes pressure to build up in the tank and force fluid out of the pump grommets.

But it will only require a very small hole in the cap to default the pressure build up. It sounds to me like to do have a leak somewhere. There are a number of plastic junctions and check valves that can fail and leak. Check on RealOEM to see where they all are.

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Wayne
2005 BMW X5 3.0d (b 02/05)
2001 BMW F650GS Dakar (b 06/01)