gprocket

I recently purchased a '97 E420 and I'm having trouble with the heat. Little or no heat on the driver's side and erratic heat from the passenger's side. After studying posts in the forum I took apart the duo valve. There was some rubber gasket pieces stuck in the upper chamber of the valve (not sure where it came from) and verified that the solenoids are working properly. But all to no avail.

Can someone explain how this system works? It looks like the water flows into the duo valve from below. Is this the back side of the heat exchanger?

I'm wondering if they are plugged (that rubber gasket haunts me)?

What else should I look for?