Your symptoms are similar to what I had when I accidently disconnected the vacuum line to the Transmission modulator.
The Modulator is the only thing that is supposed to hold vacuum in the circuit of vacuum from the main line through the vacuum valve and down to the Modulator. Note that the Modulator itself has a Cap that can leak vacuum, crack or even turn up missing.

I am not sure where you would find it in the Factory Service Manual but it has that the restricted orifice is supposed to reduce the vacuum between the orifice and the Vacuum Valve to X amount then as you depress the Accelerator the vacuum is supposed to get less and less as you get closer to full throttle again I cannot remember how much less but I think it was all the way down zero vacuum in some cases.

So the vacuum into the vacuum valve is not as high as the main line vacuum and the vacuum between the modulator decreases as you depress the Accelerator pedal.

There is also an adjustment of the linkage to the vacuum valve in the FSM.

Note that you need to T into those lines.

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84 300D, 82 Volvo 244Gl Diesel