I have an 83 300TD and recently had a similar problem. What you need to do is isolate the leak. I figured out that if I turned off my car and got out and shut the door after I locked it the car would slowly shut off in 10-20 seconds. This is because the leak was in the vacuum unlock line

To begin seeing where the leak is you must chase it from the lines running in the rail next to the driver seat. Pull up the carpeting and you can see two lines that have rubber Y's running to other parts of the car.

To test you need a Mity Vac. If you don't have a mity vac you can test by capping the lines and get the car shutting off again.

I bought the 123 Vacuum System and Door Locks Procedure video below to learn more about he vacuum system from the link below. It's not necessary, I'm a visual person and found it very helpful.

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1983 300TD - 202k with 5 speed Getrag conversion