Pulled my old espresso machine out of the cupboard where it has been since August last year when we moved.
I thought it best to run a few cups of plain water through it.
The first three and a half were gray in colour, with a thick soapy scum floating on the surface.
Then the water reservoir (freshly filled, before anyone asks) ran dry.
So I refilled it and continued.
After another two cups, the soapy scum no longer appeared, and the water appeared mostly clear.
It also stopped smelling like, well, I don't really know what it smelled like, but it didn't smell like water, because water is not supposed to smell of anything, right?
I might run another couple of cups through it just to be sure.