BX heater unit valve leaking

[freek]

Hi,
Having a wet carpet in the front right sight of my BX from 1988 I looked what is causing it. Because it is coolant it must be related to the heater unit. I first suspected the matrix itself, but having a closer look the valve meachanism is wet. It looks like a (rubber?) seal is the cause and not the valve mechanism it self. Does anyone has experience with this?
Regards,
Freek