As previously stated a low vacuum can often be the Check Valve the screws into the Vacuum Pump. However, a new Vacuum Pump I believe comes with a new check valve so one would think that would make a check valve issue unlikely.

They way to know for sure would be to remove the Tubing where it goes into the Vacuum Pump and use one of those tapered rubber adapters and push that into the Vacuum Pump Check Valve so it seals and measure the exact vacuum that the vacuum pump puts out.
If you are getting good vacuum there thest the Brake Booster to see if it holds vacuum. After that all that is left is the main vacuum line.

__________________
84 300D, 82 Volvo 244Gl Diesel