One explanation is during the atf test , a piece of carbon got dislodged and stuck in the schrader valve of the compression gauge. Take out the schrader, blow clean with compressed air, inspect, put back together and try again.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked