Dripping header tank

Hi all,

Firstly, im not sure I have used the description right, by header tank, I mean the black reservoir mounted to the wing that is filled with coolant/

Anyway, whenever the engine is running, there is a constant drip from the overflow pipe, even when the reservoir is only half full. Any ideas?

Thanks

To get out of the overflow does it have to pass the cap?

Yeah it does

Is the filler cap loose, does it seat properly in the housing, is the rubber seal intact?

It could simply be that the cap needs replacing.