Leaks aren't common on the Haldex.
Take some pictures, so we can see where the leak is coming from.
 
It's the o ring on the filter cap how much oil does the haldex hold ?? Tia
That's unusual. Might as well replace the filter, as the cover is coming off anyway.

710ml is the fluid capacity IIRC.
You fill it to the level plug, then remove 70ml of fluid to set the correct level.
 

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