For diffs, gear box, transfer box (transmission). A standard diff breather allows for expansion, as the diff heats up, a one way valve allows hot air out of the diff. If the transmission cools quickly, (ie. a water crossing) the air inside contracts like it's "taking a breath" Air cannot get in via the standard breather so it now takes the line of least resistance - past the oil seals,. This is OK but if they are under water the diff sucks in water. (oil seals are designed to keep oil in, not water out) Piranha diff breathers allow for easy access for air in and out, they are a snorkel for your 4WD transmission.
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