I have a 2005 Chevy Silverado 1500 4x4 with right around 100,000 miles on it. The other day I noticed the inner sidewall of the back tire was a little wet-looking, but it was a dry day and I hadn’t gone through any puddles. I crawled underneath, and it appears that there’s some fluid leaking around the rear brake drum—not a lot, but enough to make the inner sidewall of the tire look moist. I assume it’s coming from the axle, but how do I figure out what’s causing it, and how do I fix it? I’m new to this, but I’d like to try and fix it myself since I’m on a budget.
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Leaks around the end of a rear axle are caused by one of two things. Either the outer axle seal (sometimes called a wheel seal) is leaking gear oil, or the brake wheel cylinder is leaking brake fluid. Though it may be difficult to tell unless it’s leaking a lot, brake fluid is very thin and should be fairly clear, whereas gear oil is thick and darker. Regardless of what’s leaking, it’s important to address the leak sooner rather than later because both types of leaks can impact braking performance.
To know for sure which fluid is leaking, pull the tire and brake drum and have a look inside. Inspect the rubber boots on both ends of the wheel cylinder, and see if either one of them is wet; that’s a telltale sign that the wheel cylinder is leaking. Fixing it requires disassembling the brake shoes and hardware (take pictures so you have a record of how everything goes together), and replacing the wheel cylinder. You can also take the wheel cylinder apart and rebuild it by honing it out and installing new seals, but rebuild kits can be hard to find and a hone will cost about as much as a new wheel cylinder anyway.
If the wheel cylinder checks out, then the culprit is most likely the outer axle seal. Replacing it is a bit more of a job. You’ll need to remove the brake backing plate, which can usually be done without disassembling the brakes. Your truck’s axleshafts are retained by C-clips, so you’ll need to remove the rear diff cover and drain the axle fluid. Remove the cross-shaft in the differential carrier and then push the leaky axle inward to dislodge the C-clip. Slide the axleshaft out, then you can use a seal puller to pop the axle seal out of the housing.
While you have everything apart, inspect where the outer axle bearing rides on the axleshaft. If it’s pitted or grooved, then the outer axle bearing and possibly the axleshaft itself will need to be replaced. Sometimes an axleshaft can be saved by installing a speedy-sleeve, which is a thin sleeve that goes over worn surface of the axleshaft. Considering the mileage of the truck, even if the axleshaft is in good shape it wouldn’t be a bad idea to install a new outer axle bearing since you’re already right there. You’ll need a slide hammer to pull out the old bearing, which can be rented at most parts stores. Install a new bearing and seal, then button everything back up.
One last thing. Most likely the brake shoes have been coated with either brake fluid or gear oil and need to be replaced. Be sure to clean up the brake drum, hardware, and backing plate so that the new shoes don’t get contaminated.