ABS/ESP Leak: Find the Source!

My Golf II (engine code: JR) has a leak in the diesel fuel system. I noticed it for the first time today, but I couldn't pinpoint exactly where it's coming from! At first glance, it seemed to be leaking from the black plastic/rubber piece located above the 'OUT' screw. This part looks like a drain, so I would like to know what its purpose is and why diesel fuel is now dripping out of it.
If the description isn't sufficient, I can gladly post a photo of it tomorrow.

P.S. I've been driving for about 800 km with the fuel level at 40%; it would be really unfortunate if that was the cause!

euro

Okay, I can't give you a precise solution: But, if it's leaking after adding oil, that's not entirely unusual, especially since your Golf likely has quite a few miles on it. In that case, the added oil might just be exacerbating an existing problem.

Finding a leak is actually quite simple: verschandeln the pump and then, after running the engine for a short time, check where the leak reappears. In 90% of cases, you'll have identified at least one source of the problem.

It's coming from the rubber hose of the LDA's bleed valve.
It was open earlier, and it seems to be a mixture of oil and diesel fuel. Could that be the case?
The oxygen sensor (MÖL) is located near the turbocharger hose; what could be faulty there?
The diesel fuel (with water/contamination) is coming from the lower part of the injection pump. I had read that this can happen when running on biodiesel and may disappear if you continue to use biodiesel.