Pretty simple diagnostics. Disconnect the main vacuum tube from the pump (upper metal fitting) and connect a vacuum gage directly to it. Can buy either a single gage (used to work on 1960's gas cars) or use the gage on a Mity-Mite hand pump if you insure the pump doesn't leak. If you measure good vacuum (more than 15" Hg), your car's pump is good. If not, a new pump is >$300, rebuild kit ~$100 if you can find, or could change to the earlier 2-tube diaphragm pump since rebuild kit is $10 (has anyone?). A few have replaced w/ an electric "brake booster" pump from a modern VW or Audi.

If pump is OK, start connecting branches one at a time, while monitoring on the gage and see which connection loses the vacuum. In my experience, #1 culprit is the shutdown valve on the injection pump. When you re-hose, install a small clear filter there and when you see oil, you will know the replacement failed. Most people wait until oil is sucked up to the ignition switch and drips on their right foot, but an under-dash mess by then. The other culprit is the trunk lock actuator and 3rd is the fuel door actuator. Both can be capped off since non-essential and 95% of my other vehicles don't have those silly options. To cap both, look under the carpet on passenger side outside. You can test hoses there for holding vacuum w/ a hand pump. If a Mighty-Mite is too expensive, harbor sells a cheaper one which works fine (buy brake bleed kit). I use 1/8" silicone hose (ebay), in purty blue but also comes in black if averse to bling.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans