fwop said:
One of the factors of water getting in is temperature...
If your diffs are hot and then you plunge them into cold water the air inside contracts and pulls a vacuum. This force is much greater than normal forces pushing outward on a diff -- this is why water seems to get in even if the oil never leaks out.
In theory the vents are there to prevent this, but as many of us know it doesn't seem to work that way. Water can and does work it's way in.
Then check your vents. Air, water, and oil will follow the path of least resistance. Shock cooling the diffs drops the internal pressure, creating a small vacuum, but the air (or water) drawn into the axles has to come from somewhere. If it does not come from the high and dry vent tube, then that vent is not the avenue of least resistance, and the axle is not correctly vented.
As for axles seals passing water, anything is possible, but the amount passed through an oil seal in good condition, with axles mounted in good condition bearings, and with debris cleaned out of tube ends after every mud run, should be negligible. Not something that you could see in the diff oil as a small amount of water passed the seal due to axle shaft deflection would quickly evaporate and be vented to atmosphere. If your axles use external bearings that require greasing, as opposed to bearings running in diff lube, the seal behind the bearing may be inadequate for off road use if it leaks water. If your tubes rely on a seal located at the inner end of the tube, those seals may not good enough for use in wet off road conditions, and you may want to consider outer seals.
So far I have used Dana 25, Dana 27, Dana 30, Dana 44, AMC 20, AMC 20 with full floater, and Ford 9 axles front and rear, and none have leaked water. Only one, the Ford 9, ever leaked fluid past the tube seal, and then only because the vent got blocked with mud during a nasty mud event. Once the vent was unclogged, the leaking stopped. The rear vent tube was subsequently relocated from the upper rear fender well to the top of the roll cage.
