Series Land Rovers
Breathers in a nutshell by Alan Richer
The problem has been referred to as the "British Teakettle Effect". What happens is that the transmission and transfer case heats up and blows oil out the breather holes in the case, rather than retaining the oil. A lot of this is because the breather holes are too small, allowing pressure to build up. The other bit of it is that the breathers are holes to atmosphere with no trap for the oil internally. What an external breather does is 2 things:
1. It gives the pressure a much easier route out of the transmission, so that pressure doesn't build up. This way, it's not pushing the oil through the seals or gasket surfaces, and the oil better stays where it belongs.
2. It gives escaping oil a place to go where the oil can be recovered (gravity drip will return it to the case if the breather's routed properly). This way, any oil blown up the breather will run back down rather than out and lost. I've got one breather line running to my trans, transfer case and overdrive. It keeps things in place well, except on really long hauls...but that's tolerable.
(OVLR nl, 09/98)