The reason they can leak oil if the breather pipe is choked up is PRESSURE.
The whole axle body is filled with cold air when you start off, and in driving the whole caboodle warms up quite a lot so the air expands (Charles's Law) and that taking place in a fixed confined space causes an increase in PRESSURE (Boyle's Law) inside the axle, and that blows the oil out anywhere it can escape from.
If the breathers work freely the inside of the axle will always be at the same pressure as the outside.
It follows from this that if a slight partial vacuum were to be applied to the breather pipe it would stop anything leaking out of the axle because it would suck a little air IN instead. Just stay well clear of water if you try this.
CharlesY