You are correct---there is a vent valve on the exterior of the tank. However, there is also a "vent system, comprising a collecting tray and a tube system" together with "additional check vessels" located inside the tank. See page 47.7 IV-030/3 of the M116.96 and M117.96 engine service manual for a diagram (I don't have WIS so I have to refer to the paper manual). In my experience it is this internal contraption that suffers blockage due to corrosion/gunk and can't, to my knowledge, be cleaned. Mark