It starts out as a small oily smudge on the outside of the brake's backing plate. You notice it, but ignore it. Next time you look, there's oil and absorbed dirt making a mess all around the wheel. Yep, you've got a leaking axle seal. Let it go much longer and the gear oil will spread inside the brake drum, making a brake shoe replacement mandatory.
To fix this, your first step is to purchase a new seal. Pick a high-quality name-brand. There are some unmarked imported seals around that aren't even worth the time it takes to install them. Good seals will have a pre-applied sealer on the metal edge and a spring-supported sealing lip.
So, how do we fix it? Jack up one side so that the lube doesn't run out. Use a jackstand for support, not just the jack. But you knew that, right? Block a front tire fore and aft. If your differential is of C-clip design, you're going to have to pull off the diff cover, so you might as well jack up both sides equally. At this point it's wise to consult a service manual, because depending on design, the repair procedure varies from vehicle to vehicle. What I am going to cover here will include basic Spicer, Ford, and Chevy rearends.
Remove the tire and brake drum. You may have to back off the brake adjuster to free the drum due to a wear ridge. If so, be sure to readjust the shoes after replacement. Look at the axle flange. There's a hole in it to stick a socket and extension through to get to the four individual studs and nuts. These hold the bearing retainer in place. Rotate the axle hole to each nut and remove each one.
For those axleshafts held in place with C-clips like the Chevys, '89-and-later Jeeps, and Ford's 8.8, the rear differential cover must be removed. Once that's done, rotate the differential until you see a small bolt or screw in the side of the case. It's important to use the right sized driver to remove it, because if you strip the head, you're in a world of hurt. Removing this screw allows you to push out the spider gear's cross-pin. The gears may or may not stay in place. No big thing if they drop out, as they are easy to reinstall. Use some wheel-bearing grease to hold them in place if necessary. Push the axleshaft in and you'll see a small C-clip that slides into a groove cut in the end of the shaft. Pull the clip out and you can remove the axleshaft.
Now grab the axle flange and give it a jerk. Harder! Sometimes they're pretty tight. Still won't come out? Get a couple of feet of chain with loops big enough to fit over the lug studs. Make a loop between two opposite studs and hold it in place with the lug nuts. Put a stout bar through the loop, leave a bit of slack, and give a big, hard jerk. Pick yourself up off your back and slide the axleshaft the rest of the way out. Now is a good time to check the condition of the bearings. Excessive side movement, noise when spun, or signs of wear mean it's replacement time. It's difficult to replace them by yourself unless you have a press, so plan on another visit to your local auto parts store.
Getting the seal out can take a special puller attached to a slide hammer, an inexpensive seal puller, or you can use a pry bar, or even a hammer and screw driver. I use a metal bar with a hook on the end to pry the seal out. Above all, you want to make sure you don't damage the axletube surface that the seal presses into.
Before you slide that axle back in, check the area of the shaft that contacts the seal. Run your fingernail across it. If it catches, chances are you'll be replacing that seal again soon. There's a very thin sleeve that can be installed over the worn seal area. It's sold at your bearing supply house or auto parts as a speedy or ready sleeve. It's OK to be shocked at the price of one of these-but it's still a lot cheaper than a new axleshaft. You'll need a piece of tubing (PVC pipe slightly larger than the axleshaft works great) the length of the axleshaft as a driver. If the axleshaft has a real nasty groove in it, the seal will actually force the sleeve into the groove and in time it will leak again. In this case fill the groove with some type of epoxy and sand it smooth before installing the sleeve.
I usually pack the inner open side of the seal with wheel-bearing grease. This provides initial lubrication and prevents the spring from coming out during installation. To install the seal you can obtain a special seal driver. Using one of these makes it a lot easier to set the seal in straight. However, you can use an appropriately sized socket and extension as a substitute. A section of PVC pipe also works great. You want to make sure whatever you use is just slightly smaller than the seal's outer dimension. Too small a driver will distort the seal. Drive the seal into place making sure it's evenly seated. Once it is seated don't keep pounding on it or you will distort the seal area.
Reinstall the axleshaft, keeping as much weight off the seal as possible as the axle slides in, and evenly tighten the retainer bolts. How tight? Just about as snug as you can get them with a 3/8-drive ratchet, or you could look up the proper torque specs in your service manual. Replace the brake drums and re-adjust the brake shoes. If you let the seal leak to the point where it got all over the brake shoes, it's brake replacement time, but hopefully this didn't happen. Don't forget to check the lube level.
If you had to pull the C-clip, re-install it and the cross pin. Before you put in the retaining screw, clean it and add a few drops of a chemical locker like blue LockTite. Add a new cover gasket, refill with 75-90 weight, and you're on your way.
