Originally Posted by Cateaux

I had something similar happen on a '68 Plymouth once. What happened is that some crud actually managed to block the hole where the master cylinder sucks fluid from the reservoir. It seems like this would be less likely on a Benz, but still possible. Since the master can't take in fresh fluid, it just allows you to pump out the fluid that's in the lines. When that's gone there's nothing but air. Try pumping up the brakes and cracking and closing the bleeder like normal, but then let up on the brake. There should be vacuum in the system if the master is blocked off from the reservoir. Open the bleeder again and listen for a sucking sound, or put a tube on the bleeder and place the other end in a container of fresh brake fluid and see if it sucks any fluid into the system.

Note: I have no experience with ABS, so I don't know if it can cause a problem like that.