With a drain plug in the converter, a "flush" as it's normally done is mostly pointless and unnecessary trouble. If you're having trouble that one complete drain+filter hasn't solved, a flush isn't going to solve it either. If you're just maintaining it, change at 15K instead of 30-50K as is normal.

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'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".