The Factory Manual has rebuild instructions; at least for my car.
The 3 diaphragms are valves.
There are several models of Vacuum pumps single diaphragm, double diaphragm and the piston ones.
There is 2 rebuild kits for the Piston type Vacuum pumps. The one that you have and an expensive one that replaces the parts on the drive end of the pump. If you buy both it cost more than a new pump.

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84 300D, 82 Volvo 244Gl Diesel