fuel tank vent valve
 

Thanks to deanyel for helping me identify this(see pic) mystery part.
It is a vent valve (at the fuel tank area above the axle) and a line leads off this valve to the charcoal cannister up front. But there I found another valve. A three way vent(or purge) valve that is vacuum activated. Mine seemed to be leaking because I have many times had to wipe it dry.
All the shops online seem to have the forward "purge valve" which lists as "fuel tank vent valve. But I was only able to find both the purge and the "fuel tank" vent valve at Mercedes Benz Parts Direct.com.

I just got back from the junk yard where I wanted to find out how these valves worked. Every Mercedes car I looked at,1983 to 1990, four doors and coupes had vent valves at the fuel tank area. And they all looked to be origional. So I removed five of them. I used pliers to carefully break open these valves to find everyone had ruptured diaphrams!!
The purge valves I found all appeared to be just fine. All but one. And it was plugged up pretty good.

Glad I have new ones on the way. :pleased: