The valve itself is definitely OK (as is the cap) - no leaks from inside the valve / cap at all. The nut (joining the black hose to the silver pipe) is also tight and dry.

The leak is definitely coming from the base of the valve stem, where the valve stem intersects with the hexagonal portion of the silver pipe (see photo in earlier post).

Is there any way to fix this without discharging the a/c system and replacing the entire silver pipe? Where does this silver pipe go?

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1989 250TD Wagon 5-speed, 160,000mi ::: Dark gray metallic / black cloth
1984 190D-2.2 5-speed, 287,000mi ::: Silver-blue metallic / black MB-tex ::: SOLD