There is several Vacuum related things that can effect the Vacuum Shutoff Valve on the Fuel Injection Pump. If your Brakes are working normal there is a good chance the Vacuum Pump itself is OK.

First do an under the Hood visual inspection of the Vacuum Lined and see if there is any disconnected or there is cracked plastic fittings or rubber connectors.

Next easiest to check is the Vacuum Shutoff Valve on the Fuel Injection Pump located on the Rear Upper 1/2 of the Fuel Injection Pump.

Disconnect the Hose/Tubing; I there is black Oil there that is a indication that you have a hole in the diaphragm of the Vacuum Shutoff Valve. Plug that Hose/Tubing and connect a separete length of Vacuum Hose up to the Vacuum Shutoff Valve.

While the Engine is runnig apply vacuum to the open end of the Hose by sucking on it or using a hand pumped Vacuum Tester like the Mighty Vac. If the Engine shuts off you know the Vacuum Shutoff Valve on the Fuel Injection Pump will shutoff the Engine.

In order to find if there is a leak some where else it is easiest to bypass the other things that use Vacuum like your door locks and the Climate Control.
I do not know the specific colors of those Vacuum Lines so I will let you look them up in our do it yourself section.
The object is to leave only the Shutoff Vacuum Lines connected.

If nothing else is hooked up but the Shutoff Vacuum circuit there is anoter Shutoff Vacuum Valve located on your Steering Colum Lock. This is the one that your key actuates when you turn the Key on or off. There could be a problem with that valve or the Hoses/Tubings that connect to it.

If you have a Vacuum Gauge or a hand Pumped Vacuum Tester (it has a gauge on it) you could als check the Vacuum Pump Vacuum.

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84 300D, 82 Volvo 244Gl Diesel