No Heat in 1980 300SD
 

Not another dreaded servo issue because I replaced the original with the rebuilt aluminum body type a few years ago and has been working fine.

I recently filled with coolant cause system was low after sitting for a couple of weeks. Now I get no heat, not even on defrost setting. The recirc pump is working and the tubing to the heater core is warm to the touch but not as hot as it should be. All I get out of the vents is cool air at all settings.

Any ideas?