why would grease migrate from knuckle to diff oil but not opposite?

[e9999]

Birf: why would grease migrate from knuckle to diff oil but not opposite?

OK, been wondering about something... Thought I'd ask you whizzes!

I see no knuckle soup seeping out of my knuckles whatsoever. When I opened up the knuckle, the grease looked just fine, solid, not soupy.

Now, on the other hand, I replaced the diff oil when I got the truck (was pretty greasy) and 3000 miles later it has gotten fairly black again. Unless there was some residual grease in there, this makes me think that some knuckle grease may have gotten into the oil. That would suggest a worn seal, maybe. But the only obvious reason I can see for that migration would be that the axle housing would have developed a vaccuum because of rapid cooling. This, however, is not likely cuz I have extended my breather and left the end open. Also, wouldn't a worn seal leak both ways?

So, is there any other reason why grease would move from knuckle to diff oil and not v.v.? What? Just rotation and capillary effects maybe? Or can the knuckle develop overpressure somehow?

 

[miked]

i'd bet your last guess might be the one. the diff vents freely, but the knuckle is sealed. there are air pockets in there right? the air warms up and pushes the grease past the seal maybe? another question is, does the inside volume of the knuckle change when the wheels are turned?

 

[RavenTai]

I have observed the same, I am hoping rerouting diff vents will have helped, this afternoon I need to drain my front diff for the first time after getting rid of the evil flapper valve.

could be just residual, time will tell,

you also may have overserviced your knuckle.

a good way to tell leakage from resicual s is the axle is overfull.

yes there are air pockets, prepperly serviced knuckle chamber should be about 1/4 air
as far as I can picture the knuckle has a constant volume in turns,

 

[tarbe]

Improperly vented diffs can pull air (and grease) from the knuckle side when they cool.