Do you have A Vacuum Pump With a Diaphragm? In you info you show an 81 240D; and I thought buy that time they had Piston type Vacuum Pumps.

If it turns out you have a Piston Type Vacuum Pump check the Check Vlave on the Vacuum Pump and also as you said inspect the Main Vacuum Line.

On the firs Oil Change I did I lean on the Main Vacuum Line and cracked off one of the Nipples; I suppose worse could happen.

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84 300D, 82 Volvo 244Gl Diesel