My 1977 240D was plagued with vacuum problems.

The later W124 diesels had a separate vacuum pump under the rear seat for the door, fuel door and the trunk lock. The W123's used the engine vacuum pump for everything.

My first thought would be to check the engine vacuum pump and see what kind of vacuum it's pulling.

Are your door locks working quickly when locking and unlocking? If not, I would assume you have a vacuum leak somewhere.

Lack of vacuum in a diesel usually causes hard shifts. You may be experiencing a minor leak...

Hope this helps.
The Tenor Man

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Currently Driving.....
1987 Mercedes Benz 300D Turbo - Daily Driver.
2004 Buick Rendezvous...Smaller than it looks!
2005 Mitsubishi Galant - Not bad for a tin box...Lousey Seats
Past Vehicles.....
1994 Dodge B-250 Van- GAS HOG..Succumbed to rust!
1989 Mercedes Benz 260E - Totaled but no one injured.
1977 Mercedes Benz 240D - Loved it, but SLOWWW! - Succumbed to RUST!
1976 Mercedes Benz 240D - Same as above! -
1975 Mercedes Benz 240D - Same as above!