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.