Thank you very much, Kavouras. I'm russian, but now live in Greece (for 10 month only, so speak Greek only a little). I'm a member of audi club russia, and russian audi owners have the same problem also. 90% of audi dealers in Russia says that this is normal, but I think it's not. Other 10% of dealers consider this as a problem and say that the problem is exacly in pressure hose, like you told. They replase it to the new hose with bigger inner diameter and this solve the problem. From the other hand, people, who haven't done this replacement say, that thay already drive with such problem with 50-60.000 kilometers and more, and the problem don't become worse (nothing happens to the car). That means, if this trebbling of the stearing wheel doesn't annoy you, you can easily use your car.
P.S. great that you have audi club in Greece also, so it will be a pleasure to meet someday for drink