Unless you can find an external leak I would suggest that you are getting excess crankcase pressure from somewhere.... usually either turbo "blow by" coming from the compressor stage of the turbo or from piston ring wear / failure or a cracked liner.... but if a liner is cracked then you are also likely to be losing coolant and emulsifying the oil.
You don't mention the exhaust... are you getting blue smoke out of the exhaust suggesting that the engine is burning the oil.
Perhaps someone else on the forum knows whether the G9T has an open crankcase breather system or a closed one. In an open breather system cleaning out or changing the breather pipe, as you have done, would usually solve the problem. However, in a Closed Crankcase Ventilation system there is also a filter and a relief valve (some engines with CCV also have a sensor).
If the G9T has CCV then you are certainly looking along the right path for a relief valve.... but where it can be found I don't know. Sorry I can't be of better assistance but as you had not so far received any response I thought at least I'd give what I can.
Any German speaking members on the forum able to determine whether this issue is covered on www.espace-freunde.de?