Finding and Fixing Leaks

Differential Fluid Leaks

Differential fluid often looks just like motor oil. Once again, you’ll have to put your eyes on the leak to determine its origin. Usually, if you have a leak at the rear of your vehicle and it’s rear-wheel drive or all-wheel drive, it means your differential is leaking. Differential oil usually has a distinctive smell. So if you see oil from a leak and it has a distinctive odor, it could be a differential fluid leak.

However, differentials don’t always leak from the differential itself. Sometimes the outer axle seals fail and fluid leaks into the brake assembly. You might not see a leak of this type right away. What you might notice, though, is an increase in braking distance or a noise coming from the rear brakes when they are applied. You can sometimes spot this trouble by looking at the back of the backing plate. If you have an axle seal leak, the backing plate is often covered in oil. Keep in mind this could also be brake fluid, so you might as well remove the wheel and brake drum (if applicable) and inspect the brakes. If the wheel cylinder isn’t leaking, then it’s likely the axle seal is bad and needs to be replaced.

FYI, when replacing axle seals of this type, you often need to replace the axle bearing at the same time. Be prepared for this eventuality should you find you have a leaking axle seal. If you find that your leak is in the front of the differential where the axle joins the assembly, it’s likely your pinion seal is leaking. Pinion seals aren’t always easy to replace, so be sure the leak is bad enough to be worth fixing before you dive in. In addition to replacing the seal, you might also need to replace a crush sleeve installed in the differential that helps set the proper preload on the pinion. If this preload is not correct, then you could damage the differential. Be sure to check the service procedure for your vehicle for torque specs and proper procedure before you commit to replacing your pinion seal. It might be more involved than you suspect, and it would be good to have that information before you get in too deep.

Also, know that a limited slip differential takes a special fluid or additive to operate properly. Sometimes limited slip differentials are identified by a tag bolted to the outside of the unit. Other times, it might be stamped into the cover. Either way, be sure to use the correct fluid, especially if you have a limited slip differential. If not, you could damage the differential and that can get expensive. FYI, when a limited slip needs a fluid change, it will often make a noise when you make a turn. You might also notice this if the fluid gets too low.

Up to this point, I’ve been talking about the traditional straight axle. These days, there are a lot more configurations than just a straight axle for differentials, not to mention the differentials used in front wheel drive or all wheel drive systems. That said, always consult your service manual for the correct procedures and torque sequences for your application. You’ll also find information about what type of fluid should be used.

Before you dive into repairing your differential fluid leak, be sure to do your homework and know what you’re getting into. If you don’t, you could be in way over your head before you know it.

Transfer Case Leaks

The drill is pretty much the same with transfer cases. Transfer cases are found on 4WD and AWD vehicles. Normally located behind the transmission or sometimes contained within the transmission itself. A transfer case splits the power coming out of the transmission and sends it to the front and rear wheels. It’s normally a self contained unit and therefore has it’s own fluid. A leaking transfer case could mean that you’ve had a seal failure but it could also mean that there is an issue inside the transfer case itself. If a bearing goes bad inside a transfer case, it could cause it’s output shaft to wobble and thus causing a leak at the oil seal. This is true for many oil leaks if I’m honest. So if you have a leak that keeps coming back after you replace a seal, you might want to verify that the part that it’s sealing against isn’t loose. If it is loose, this could be the root cause of the leak. Unless you fix the bearing or bushing problem, the component will continue to leak no matter how many new seals you throw at it. Consult your owners manual or service manual for the type of fluid your transfer case uses.

Brake Fluid Leaks

Brake fluid leaks can be dangerous. If you have a brake fluid leak, address it immediately. Brake fluid leaks can happen just about anywhere under the vehicle. Brake fluid is usually a clear amber color. As it gets old, it turns brownish. As with most leaks, you need to put your eyes on the source to know for sure what the source is. You might start by checking at the wheels. Most leaks occur at the calipers or wheel cylinders. It’s usually best to remove the wheel and get a firsthand look at the leak. In the case of wheel cylinders, remove the wheel and drum to gain access to the wheel cylinder. Then pull back the dust boots and look for fluid leaks. If fluid comes pouring out, replace the wheel cylinder.

I don’t recommend rebuilding these. Often the bore gets rusted and pitted to the point of being unusable. Just replacing the seals in one of these won’t do the trick. You’ll end up throwing your time and money into it only to have the wheel cylinder leak again. Wheel cylinders are cheap enough; I recommend you just replace them if you find one that’s leaking.

Something to note: A small amount of fluid when you pull back the boot is not out of the question. In fact, it might be normal. If fluid comes running out when you pull the boot back, replace the wheel cylinder. If you just see a few drops in there, I would monitor the situation and see if it gets worse. Use your judgment here. If you’re in doubt, replace it and be done with it.

In addition to the calipers and wheel cylinders, check the brake lines themselves. If your undercarriage is covered in rust, it’s a good bet your brake lines are too. If they corrode to the point of leaking, it’s best to replace them. This is often easier said than done. When brake lines are installed at the factory, the body is often not installed on the chassis. You don’t have that luxury. You might find yourself having to bend the line into place to install it. The takeaway here is that not all brake lines will be easy to install; in fact, more often than not, they’re a real pain. Just be ready for that if you have to tackle this.

One note on brake lines. I’m often asked about purchasing brake line and bending and flaring it to suit. This is fine, IF you have the right tools. Andy by the right tools, I mean quality tools that do their job correctly. I can’t tell you how many times I’ve used cheap flaring tools only to find they weren’t up to the task and as a result created a flare that leaked. Quality tools cost money. You might be able to rent or borrow them, but the best tools have the best results when it comes to doing this type of work.

An alternative to purchasing or borrowing expensive flare tools is to purchase pre-made lines that are already flared and have the fittings installed. The line itself will not be bent to fit your application; you will need to do that task. But if you purchase the correct length, all you need to do is bend it into place. If you go this route, you might bring your old brake line to the parts store to be matched up with a line that will work for your application. This is a nice alternative if you’re faced with this problem.

Other brake fluid leaks might not be so obvious. For instance, if your master cylinder is leaking, it can leak internally OR into the brake booster. If this is the case, you won’t necessarily see where the fluid is going. You might feel the effects, however. Mostly what you’ll notice with a master cylinder leak is that your foot slowly sinks to the floor when you’re at a stop light. The vehicle might also begin to creep forward when the brakes are applied and you’re in gear. If this happens, take a good look at the master cylinder. I cover these issues in detail, as well as how to bleed a brake system, in the Brakes Article.

Be sure to check your brake booster for signs of fluid contamination if you find you have a bad master cylinder. You can sometimes get a light and a mirror down in there to see what’s going on. If you find fluid in the brake booster, you might consider replacing it, as it can cause issues down the line. When brake boosters go bad, they often give you a very hard brake pedal and the vehicle is difficult to stop. This is because the brake booster does just what it says: assists in boosting brake pedal output. Without it, your brake pedal gets very hard because there’s no assist. See the above article on brakes for more detailed information.

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