I have a van, Mercedes-Benz L406D from 1975, and I was venting the brakes after replacing the brake fluid, and the first two wheels went OK - these where both back and front right wheels. But the venting valve of the front left wheel was so rusted and not un-screwable that I broke the valve. And now I must somehow remove the half of the valve that is stuck in there. I thought I would unscrew the whole wheelbase, I mean the fixed part of the wheel, where brakes are, to make things easier, to be able to do it with an electric drill, which would not fit behind the wheel. So I took of the wheel, and then started unscrewing 4 nuts that I thought are holding the part.

Attached is a photo of the wheel base with 3 of the 4 nuts I tried to unscrew marked with green arrows. The 4-th nut is not visible on this photo because its hidden behind a piece of metal.

And now my problem: the 4 nuts, which are marked with green arrows, they unscrewed just a bit, so now they turn around quite easily with the 17 mm key, but they do not come off at all! They just rotate around, and the screws which are inside the nuts, they also turn around with the nuts, but nor the screw comes out, nor the nut comes off. And whats even worse: I can not even screw them back to the state when they where fixed, because turning them clockwise, also does nothing, besides turning - it doesn't fix them in place.

The red arrow points to the venting valve that I was trying to unscrew and I broke.

The blue arrow points to something that I have no idea what its for, and would be curious to know.

Any help with getting my wheel back to a working state is welcome. Even better where I can get/download/buy a service manual for this van, so I can read about such things explained? Maybe someone has a paper copy and can scan it and send me the scans?