I changed the fluids in the front and rear diffs, now I have a leak at the rear pinion seal. After searching on here I checked the vent tube and it's clear, and I read that changing the fluid can cause the seal to leak, my question is why? Just curious I guess, kinda a pain that I have to take it apart and put in a new seal because I was doing routine maintenance.

 

I have a hard time believing that changing your diff fluid will cause your pinion seal to leak. I've changed my diff fluid several times and it has never leaked. I have on the other hand see people add fluid to a low diff and have it leak, changing the fluid on that same low diff would leak as well. But its most likely a worn seal that has leaked, a damaged seal from an overheated diff, caused by low fluid. Could even be the additives in the oil you add to the diff cause the seal to slightly shrink and then it leaks. But, a fluid change in and of itself will not cause a pinion seal to leak.

 

I have experienced the same problem. Also I have intermittant leakage from the seal, more if I overfill the diff. I have made the arrangements to get the seal replaced only to have it stop leaking. I panicked thinking my diff was dry, but the fluid was down only 1/4-1/2" below the plug which is normal.

 

Hate to say it but over filling is the only way. Too much is almost worse than too little.

 

If the leak started right after the fluid change, the seal was on its last legs anyway. The new strong additives in the fresh oil attacked all the gunk built up around the seal lip and the leak started that would have started soon enough anyway.

 

I don't know the spec on the rear diff, but on the front diff it's clear full when the fluid level is 3/4" below the fill hole in the diff cover. I've overfilled mine and had it leak until it reached the 3/4" mark. I'd assume it just makes too much pressure for the seal.

 

I don't know what Dana was thinking in their design of the fill hole but their specs are a maximum of 3/4" below the bottom of the hole in the front and 1/2" in the rear.
I guarantee leaks eventually if you fill to the bottom of the hole.
Sometime later, maybe 2000, they lowered the hole to make it easier.
If you don't believe look at the specs in your owners manual.
I've "fixed" the problem in several folks trucks by simply sucking some fluid out.
They also will stop leaking once enough fluid leaks out to get to the correct level.

 

Well, I think for now I'll just keep a close eye on it and see if it cures itself once it gets to the right level. The 3/4" below the plug is silly, It seems pretty standard to fill right to the plug, didn't think twice about it.

 

It caught by surprise me the first time a changed mine also.
Wasn't till a friend pointed the specs in the manual out to me that I believed it.
3/4" below the hole comes out to over a quart overfull.
That's a lot when you think about it.