I had the same problem with my 535. Not sure if the reservoir is in the same position on yours but its hard to see which power steering hose leaks. I took off the stock bmw clamps and put on new hose screw clamps. Then cleaned the area with gunk and rag. The next day I could see the hose from the reservoir was the leak and replaced it. I still have some hose left if you need some. I am glad it was the low pressure hose needed only one foot and two clamps. If you find the leak is the same email me and I will send you enough hose to replace it.