Here's a thread where the OP's engine would not shut off, and how he rebuilt the pump on the vehicle. You've obviously found the procedure in the manual.

http://www.peachparts.com/shopforum/showthread.php?t=266860

good luck!

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)