Recently I went through this problem on my 1983 300TD wagon. First you need to have a Mityvac vacuum pump (the pump has a vacuum gauge). The yellow/grey stripe vacuum line is the one that provides vacuum directly to your vacuum reservoir. Locate the vacuum reservoir (in the trunk located under the parcel shelf), pull off the vacuum line from the reservoir and plug the line with a golf tee. Locate the same yellow/grey stripe line under the hood and using your Mityvac apply 20" of vacuum to this line. This line should hold the vacuum for at least 1 hour (hopefully longer). Next check the reservoir itself by applying vacuum to the reservoir (it will take several pumps of the Mityvac to achieve 20") and see if the vacuum holds for at least 1 hour.

Next, using your Mityvac, disconnect the vacuum lines from the vacuum line that goes from the vacuum pump to the brake booster. Hook your Mityvac to the vacuum T that is close to the brake booster and cap off the other T. Start the engine, you should see a minimum of 20" of vacuum. If anything below 10", you need a ew vacuum pump, or new pump check valve.

Do these checks first, post your results, and if needed we can provide further checks.