Hello Hans,
is it really the brake pads dragging the rear wheel? Usually with squeaking sound while turning the wheel? And you can't insert a thin thickness gauge between disk and brake pads (after a short riding)?
The final gear makes the wheel quite sluggish too!
But if it is the brakes, you might have made an assembly error. Perhaps reconsider your steps.
If you can speak German, the German Kymco Forum
http://www.zzip.de/forum/ might be interesting for you. Its admin "Meister Zip" is a renowned authorized Kymco dealer and there are many (German and) international users, including some from the Netherlands.