There's a flay alloy washer, similar to the sump plug, that's supposed to seal the breather..................
Coat it with RTV sealant before screwing it back in and it should stop the weeping.
Sometimes though the oil leaks out of the breather itself. The alloy outer is crimped on to the plastic base.
Later bikes had an all plastic one and I think that uses an o-ring to seal. Could swap over to ones of those or fit a nice alloy one
