No general article like this is enough guidance to safely work on your own vehicle. Before assuming the risks yourself, make sure you consult the appropriate material specific to your vehicle or a professional. Read on to get a very general overview of how brake fluid works and what replacing it entails.
The Basics: What you need to do to stay safe
Here's the need-to-know stuff. First of all, your brakes don't work without brake fluid. It's a hydraulic system. Brake pressure pushes fluid down to your brakes, pushing the pads onto the rotor (these are called disc brakes, because the rotor looks like a disc). The rotors are attached to the wheel hubs, and spin as the wheel turns. More pressure causes the car to slow down. No fluid, no pressure. You have an emergency brake (also known as a parking brake), but it's often too weak to really stop the car in an emergency. So if you or your mechanic spot a brake fluid leak, it's a big deal. You should fix it immediately, and you should not drive it until it is fixed.
If there are no leaks, you may still have issues. Brake fluid, over time, goes bad. It absorbs water from the air, and this causes gunk like rust to build up. Water also has a much lower boiling point than brake fluid. If you are braking really hard, the water might boil in the lines carrying the brake fluid to the brake components. If that happens, you could "lose" your brakes – the pedal might flop onto the floor, and the car won't stop.
The brake fluid change interval varies, but a good rule of thumb is every other year. Ask for a brake fluid change when you get an oil change after the right amount of time has passed. It's usually not expensive if you have a typical braking system.
Lastly, if you're topping off a low brake fluid reservoir, only use brake fluid from a fresh container. Brake fluid starts to "go bad" almost immediately. Luckily, regular fluids are inexpensive. Buy only what you'll use relatively soon. Brake fluid lasts for about 2 years in an unopened container.
Can you change your brake fluid yourself?
Yes! Generally, you don't need special tools. But you need to have some basic experience working on cars and you should read a dedicated tutorial from a repair manual that's specific to your car. Make extra sure to pay attention to the type of brake fluid and the brake bleeding procedure. It's not difficult to do, but it needs to be done right to be safe.
What are the different types of brake fluid?
Since it's such an important safety system, owners often wonder what they can do to make their brakes work as well as possible. We're used to seeing "premium" fluids that go in our cars; fancy synthetic oils and special additives.
Generally, brakes are not that fancy. For most people, clean fluid that's changed regularly is all you need. For folks who drive their cars on the track or in other severe situations where brakes are used extremely hard, there are some specialty brake fluids that are better. But for regular drivers, these are overkill.
Let's run through the common brake fluid types. Remember, you will want to use what's recommended in your owners manual. Blending brake fluids is generally a bad idea!
DOT 3: Usually formulated with a glycol-ether base. The minimum dry boiling point (pure brake fluid) is 401 F, and wet (measured with about 4 percent water mixed in the fluid) is 205 F. It's appropriate for regular vehicles used for typical purposes, like commuting.
DOT 4: Very similar to DOT 3 but with additives that increase the minimum boiling points. DOT 4 fluids have a superior dry boiling point, but require more frequent changes. DOT 4 fluids designed for racing or performance cars can significantly exceed the minimum boiling points – they're sometimes referred to as Super DOT 4 fluids. Regular DOT 4 is appropriate for regular vehicles. If it's the specified brake fluid type, the factory replacement interval will take into account DOT 4's increased water absorption rate.
DOT 5: Not compatible with any of the other brake fluid types. See below. DOT 5 is doesn't attract water, acts as a rust preventer, and doesn't harm paint. It's also extremely expensive, and is outperformed by specialty DOT 4 fluids. Unless your car is designed to run DOT 5, or you have a very specific reason to run it, you can safely ignore this fluid.
DOT 5.1: It's similar to DOT 3 and 4 chemically, but has the same minimum wet and dry boiling points as DOT 5. It has a lower viscosity, which is required by some vehicles. DOT 5.1 is not "better" than DOT 4 for any given application.
Can you mix different types of brake fluids?
The short answer is no. Use only what's recommended. Change it at the recommended intervals to prevent any issues.
The longer answer is that DOT 3, DOT 4, and DOT 5.1 are technically compatible. If your brake resevoir is dangerously low, in a pinch, one could be substituted for the other. Don't drive it that way for long. You might have a fluid leak. Take it to a professional to flush and refill the system so it has fresh, clean fluid of the correct type in it.
DOT 5 fluid cannot be mixed with any other type. It's based on silicon, rather than glycol-ether. If you have a car that requires DOT 5, that's it. Don't put anything else in. It could damage your brake system. It may be possible to convert your car to use DOT 5, but let a professional do it.
Who named two incompatible fluids DOT 5 and DOT 5.1?
Whoever it was, they get low marks for foresight. It's very confusing and the consequences are serious.
Why do you need to change your brake fluid?
DOT 3, 4, and 5.1 brake fluids are hygroscopic. That means they absorb water from the humidity in the air around them. Even with the brake reservoir lid tightly sealed, eventually enough moisture will contaminate the fluid.
What's so bad about water in the brake fluid? Well, brake fluid is incompressible under pressure, and since it has a high boiling point, the heat from your brakes isn't enough to cause it to boil. But water has a much lower boiling point. If the moisture-contaminated brake fluid starts boiling, it creates gas bubbles. Gas is compressible. So when you step on the brake pedal and create hydraulic pressure in the brake system, instead of that force being transferred to your brake pads in order to grip your rotors (or drums) and slow your car down, that force is wasted compressing that gas.
In more practical terms, it could mean your brake pedal just sinks to the floor without stopping the car. That, friends, is very bad.
Water in your brake system can also cause rust, which can gum up the small passages in the brake lines or brake hardware, and cause your brakes to work improperly or even drag – a situation where the brake pads don't disengage from the rotor or drum, creating friction and heat, and perhaps causing even more damage.
Luckily, rather than guessing or testing to see what the moisture content is, you can just follow the manufacturer's brake fluid replacement requirements. That way you know you have clean, effective fluid that won't let you down when you need to stop. If you drive something that doesn't have a recommended interval, the rule of thumb is every two years. For high performance cars that see spirited driving or time on the track, try to replace it every six months. For pure race cars, brake fluid is generally replaced every race. As you can see, the harder you'll be on the brakes, the more frequently you'll want to change your fluid.
What is bleeding your brakes?
If you're interested in replacing your own brake fluid, you'll need to bleed your brakes as part of the process. Bleeding is a process by which you push new brake fluid through the brake lines, driving out old fluid and any gas bubbles that have formed. Once the fluid that is drained off is clear and free from bubbles at each corner, you're done.
The process itself requires a few things. You'll need a catch container, a wrench to open the valve at each brake caliper, and either a friend to operate the brake pedals or a pressure-type bleeding tool. The specifics of the process are more than we can go into here, but we encourage you to read or watch a tutorial specific to your vehicle before starting. We have a great general overview here to give you an idea of what to expect.
Running the brake reservoir dry or opening the bleeder valves too far can actually introduce more bubbles into the system, which could be a serious safety problem. It's not hard as long as you follow the directions carefully and pay attention.
If you're unsure, take your car to a professional to have it done right. Brakes are your most fundamental safety system, and you don't want to do anything that might compromise that.
Do I need a brake fluid flush?
A "brake fluid flush" is a service that's sometimes offered by mechanics. The name of this service makes it sound like they're pumping the brake system full of a powerful detergent to totally clean out the system. The reality is, a brake fluid flush is the process that's described above. New fluid is forced into the system, pushing out the old fluid and any bubble, rust, or debris in it. That's it – and it's all you need, too.
Now, professional mechanics do have fancy pressure-operated systems that make it quick and easy. And as long as you're not flushing the brake system unnecessarily – more often than the recommended interval and taking your driving style into account – it's not a bad idea. Just make sure the service is reasonably priced. You can compare repair costs in your area using a tool like Autoblog's Auto Repair Estimator. There are also third-party repair estimators you can use as well.