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Dear Brian S,
Good a.m. and may this find you well. Has this always leaked a bit?
I suspect that the metal surface is not perfectly flat, or, at any rate, not flat enough to ensure a good seal. If the metal surface is "out of flat" -- from the left-hand edge of the big hole to the right-hand edge of the "o" ring in the photo above -- even if you install the "o" ring properly and bolt up tight, neither you - nor anyone else - can get a complete and so lasting seal.
In the last photo, immediately above, the metal at the edge of the bigger hole, closest to the block, seems to be rough.
If you can get a small straight-edge (a machinist's tool, lapped to ensure a "perfect" straight edge) and lay it across the openings, I suspect you'll find a small gap between the edge of the tool, and the edge of the openings. If this guess is correct, then you'll need to polish the high points until the gap disappears.This gap may be a bit too wide for the "o" ring to span, so oil bleeds around the "o" ring.
If the surface is polished, when you bolt-up, you'll get a tight seal. Since it is infeasible to remove the engine and use a milling machine to remove metal, a hand-held stone will have to do. It will have to be laid flat across the opening, and moved side-to-side, with consistent gentle pressure. I'd stuff the openings with rags to trap abrasive particles (and wash the area with brake cleaner afterwards, using "Q" tips if necessary to get it surgically clean.
This is likely to take some time and lots of elbow grease. Alternatively, if the breather box is a relatively cheap part, you might want to shape the top surface of the breather box, to conform to the metal.
Unless the two surfaces - the metal and the plastic - are perfectly flat or mated, you can't get a lasting seal, because the combination of heat and pressure will displace any but the toughest sealants.
I do not know if it is permissible/safe to use a viscous, copper based sealant in this location. Even that may not stand up, if the gap between the surfaces is too large, because plastic - subject to heat - will expand and break the seal.
Hope this helps.
Yours faithfully,
spook
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