You will need to find the vacuum spec for your particular Engine.
On My 84 300D there is the Main Vacuum and between that and the Vacuum Valve on the Fuel Injection Pump is a restricted Orifice that reduces the Vacuum to the Vacuum Valve.
As previously stated the Vacuum Valve itself has an adjustment.
If the Vacuum Valve is working the Vacuum will be high when the Accelerator Pedal is not depressed and as you depress the Accelerator Pedal the Vacuum gets lower.
It also is not unknow for the Vacuum Modulator Cap to be cracked or missing and the Modulator itself to be not holding Vacuum properly.
I did not give any vacuum specs because yours are likely different.
This article is made more for the Cars but the same idea applys.
Transmission Vacuum and adjustments
Mercedes-Benz Transmission, Steve Brotherton, ImportCar, February 2002