Mystery brake leak at new MC

Bewy said

Those stop plugs should have a copper washer under the head. The copper being soft, will compress & form a seal.

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I have to disagree, a washer for a "plug" that only goes in so far has no application, at least in this case (same thing goes for the coolant drain plugs in the cylinder blocks).

I see the '65 Factory Service Manual does not even picture the plug in the parts blow-up diagram.