About 12,000 miles ago my rear pinion seal started leaking. I put a speedy sleeve with a matched seal. The other day it started leaking again, so I decided to check the gear oil level in the morning after it cooled off and when I pulled the fill plug there was a sucking sound when the plug was loosened. So I realized that the vent was plugged it was not in the hose but the actual fitting that vents the housing. I unclogged the fitting with some brake cleaner and put it back together and filled it as much as I could on the hill in my drive filling it about 1/2" above the plug then quickly put the plug back before it could drain back out(I dont know why!). The problem is it is still leaking......so did I just overfill it or did the pressure that builds while driving because of the clogged vent ruin the new seal? BTW the leak seems to be slowing but I did wipe it down to check and it is still seeping only while(after) driving at highway speeds.

 

The pressure may have rolled the lip of the seal out but it may stop leaking after a while. Mine was clogged and caused a wheel seal to leak but it stopped leaking after I unclogged it.

 

What he said. You might have to replace it if it doesn't pop back into shape.

The correct fill level for the D70 is 3/8" to 1/2" BELOW the bottom of the fill hole. Excess usually winds up in your brake drums.

There's a link in the sticky to axle vent cleaning, seems it's common for them to plug up.

 

The seal on my Ford has leaked slowly, for years.

I found the vent is broken off at the axle-housing, and rusted/plugged solid.

Due to the huge rear fuel-tank being about two inches behind it, I can't get anything serious enough to remove it back in there.

The next time I have my cover off, I am going to invent a vent in it.

 

Make sure your pinion bearings aren't worn out. That's the case with mine, as it was a farm truck that spent more time with a gooseneck full of cattle behind it than it did without. What's odd is that the rear diff doesn't whine or clunk under throttle transition.......but the distance I can move the pinion forward with the driveshaft disconnected is pretty scary. It's worth checking, as that will wear out seals quickly.