4.8 expansion tank - level sensor leak

[svvg]

Grateful for some advice please. I’ve got a 2004 e53 x5 - with a 4.8 n62 engine. 108,000 miles. I’m losing coolant - topped up 1.5 litres yesterday and it’s all gone today.

First thought was the valley pan gasket transfer pipe, but upon closer inspection it’s coming out of the base of the expansion tank (yep. Lucky bstrd....) - I think from where the level sensor fits. The tank’s empty down to the level of the sensor (as residual fluid left in the base of the tank....) - and when I removed it to check, was really surprised that there’s no o-ring on the sensor? - So am not sure how it’s supposed to seal?

Anyone know anything about this? Is there supposed to be an o-ring? Real oem doesn’t show an o-ring on the part...

Or have I got a bigger issue with too much pressure building up and it’s pushing it past the sensor (which might otherwise be capable of sealing perfectly....).

Many thanks!!