Removing heater core valves in a 111

[wwheeler]

Anybody know how to remove the heater core valves in a W111? I have the lever off and have marked what position it is in. I have screwed a long 4mm screw into the threaded hole. When I pull on the screw head, the valve moves just a bit but seems to be stopped by something. I have heard there is a retainer but can't seem to find it.

Any suggestions? BTW, the valve is not seized and will rotate. I am replacing the o-ring because it leaks a little.   

[glenn]

(Digging into memory)  The valve(a rotating plug) has a iron whatsit that rotates in a slot in the body.  The whatsit must come out before the plug can come out and the o-ring can be replaced on the plug.