Remove the vacuum mainline where it attaches to the vacuum pump. Unscrew the fitting that is in the vacuum pump. That is a check valve. Look inside and see if the parts of the valve have fallen out of it or if it is full of sticky gunk.

If it is full of sticky gunk hose it out with WD-40 to clean it. I had that happen and the WD-40 fixed the issue. Do not use brake cleaner as the valve inside is plastic and brake cleaner makes some plastics sticky and soft.

If the check valve inside parts is gone you need a new check valve. Thos parts fall into a passage in the vacuum pump, but I have never read of them damaging anythng in the vacuum pump.

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84 300D, 82 Volvo 244Gl Diesel