Differential pinion seal leaking after replacing pinion seal

I have a 742 case with the large u joint. I took my differential to a shop in December to have the pinion oil seal replaced. No other work was done. Just the oil seal replaced. After installing the differential back on the vehicle, it started leaking again, at the oil seal. So my question is: assuming the differential shop did everything correct when they replaced the seal, why would it be leaking again, and so soon? My theory is that the 50 plus yeary old pinion yoke has a ridge or groove in it and no matter how many oil seals I put in it, its going to leak. Am I wrong on this theory?

Not unusual for the yoke to get seal cut. They make repair sleeves if it's deep.

But, could be leaking through the splines too. Or even around the outside of the seal if the housing has imperfections.

I wouldn't take it back to them if they missed these things. Good news is you can fix it yourself, there's no voodoo in a 742.

Not much of a shop if the yoke was grooved and they didn't fix that. There is this thing called a speedi sleeve that is designed to fix exactly that problem.

https://www.rockauto.com/en/catalog/dodge,1968,coronet,6.3l+383cid+v8,1081831,drivetrain,differential+pinion+repair+sleeve,13976

They can weep oil down the pinion gear splines and past the nut and washer. Most of the time it is the seal, but that is another potential leak source that gets overlooked. When the whole area is full of lube mess it can be hard to tell sometimes.

Hi Mopar , Did Shop use a Original style 742 MOPAR # 1828719 leather seal , or the rubber Mopar # 2404135 same as SKF or CR #18912 ? Need to know to answer . Ronnie

Is the axle vent clean and open?

On a 68 charger, 8 3/4, where would the vent plug be?

Same place it is for every 8 3/4 built. The bolt holding the brake line junction block is also the vent.

https://www.doctordiff.com/mopar-rearend-vent-bolt.html

https://www.forabodiesonly.com/mopar/threads/8-3-4-rear-axle-different-vent-positions.341891/

Exactly , so where is it actually leaking from, behind the yoke shield or at the u joint ?

When replacing a pinion seal, some people don't pack the garter spring with grease and the spring can pop off when the seal is driven in.

This can happen to any lip seal with a spring. I've seen folks install torque converter and shove the spring into the pump!

Behind the yoke shield.

I looked on the receipt. It just says pinion seal replacement.. Probably the cheapest pinion oil seal was installed.

leather seals require a bit more attention to detail before installation. They need to soak in a light oil. It makes them pliable and supple. Otherwise they fail quickly.

Sort of like using a leather accelerator pump cup. I usually throw that in a light oil to soak when I start the carb tear down. By the time I am ready for it it is ready to be installed.

I have a similar situation. I took a Dana to the local ring & pinion shop back in 2015 to have new gears installed as well as axles and seals. Put the rear end in my car (which was in the process of being restored so it wasn't being driven yet) and in a very short time I noticed drips on the garage floor. Checked and found fluid leaking out of the pinion seal. I didn't have the spring perches welded on yet so I just tipped the rear end up so no more fluid would leak out. I still don't have the car back together yet but once I get it on the road I guess I'll have to take it to the pinion shop and get another seal installed.

Do be aware that welding the perches without a jig in place can tweak the tubes.

On the pinion seal, Just do it yourself. Pretty easy except for torquing the pinion nut. And make sure to use red loctite on the threads and stake them when done