I just solved this problem with my 20 feet of heating lines running to the trunk of the car for my veg oil conversion. The thing that worked best was to go for a short drive and really rev the car. That seemed to blow all of the air out of the system. You should make sure that the radiator is full before you try it, and it should be lower when you are finished. Other suggestions I got were to park nose up on a steep hill and run the car, and use a garden hose to fill the lines (in your case the heater core), but the one that seemed to work best was driving at high RPMs for a few minutes.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine