On a 1982 300D, which is the high side and which is the low side service port for charging the A/C? There is one port that goes across the front of the engine on top on a hose just behind the fan and there is one underneath on the passengers side of the engine.

The one that goes across the front of the engine is the low side (suction).

Thanks, that is what I suspected but the problem is, I just installed a new compressor, and when I try to put freon into the system using that port, I get a lot of back pressure. There is no suction at all. Any ideas?