You can't expect the interface of the cap to the neck of the exp tank to be totally dry. If pressure is being vented via the cap, some wetness there is to be expected. Do you have a vent hose at the neck of the exp tank? Whatever coolant that is vented normally go through that hose.

I see you prefer trial and error by replacing parts rather than diagnose with test equipment?

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked