OK, I finally got to spend a couple of hours on this problem.
I took the pump out which is located right in front of the two valves and it was not turning very well. Interesting machanism where a small impeller is driven through a magnet. I cleaned it and it works well now. But it did not change the condition.
I checked all water lines coming and going to the valves and pump and all had rusty water sitting in them, which I guess is due to it not working for a year now. I was able to get water out of all lines but one. There seem to be four lines going towards the firewall the two lower are larger in diameter and the two top ones are a bit smaller.
The very top hose (it runs on top back at the firewall) seems plugged. Nothing going in or coming out.
I assume that is not normal, can someone verify if that is normal? How can I troubleshoot this?
Is there a flow diagram?
'07 ML320 CDI (398 ft lbs / 1400 RPM)
'91 928 GT Supercharged