Be sure it it the Vacuum pump that is causing the failure. (Be careful where your put your hands while the engine is running.) Disconnect the big vacuum line that goes to the brake booster at the vacuum pump and put your vacuum gauge there and test it. If no gauge put your thumb over the connector hole and see if you can feel any vacuum. If you are not getting vacuum from the connector try spraying some WD40 type stuff down the connector hole and see if it will clean out any gunk that might be blocking the valve that is down there.

The rebuild kit is sold in 2 parts. The part with the piston seal (if you have a piston type vacuum pump) and valves/O rings and stuff can be as low as $43 + shipping. The other (more expensive) kit has the mechanical parts parts like the rocker and the bearing.
Have you thought about taking it a part and having a look inside. It could be that the little valves are all gunked up and being held open. I might at least buy you some time till you decide what to do. A new pump is in the $300 range.
I took a look at mine and removed the power steering belt for easier access. The only problem I had is I sheared off 2 of the screws that held the cover onto the vacuum pump body. I needed to heat the rest to get them off. Since I did not have a new seal ring for the pump cover I used the old one with some silicone sealant when I assembled it. It is working fine.
Added later: The complete rebuild kit seals + rocker arm and relate parts at one place is $265 + shipping.

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84 300D, 82 Volvo 244Gl Diesel