Brake fluid can and does go bad. Despite being one of the most crucial components for safety and performance, changing brake fluid is one of the most neglected maintenance tasks.
In fact, people rarely change their brake fluid before it goes bad. They often wait until they hear the grinding sound from the wear indicator on the brakes, then change the brake pads and bleed the brakes in the process. But the truth is, regularly maintaining your brake fluid is essential to ensure firm, reliable braking.
Why Brake Fluid Goes Bad
Brake fluid is hygroscopic, meaning it likes water just like a sponge. Brake fluid absorbs moisture from the air over time through the seals, hoses and even the brake reservoir when it’s opened. This absorbed water lowers the fluid’s boiling point, which can lead to dangerous brake fade and a spongy pedal feel.
Dry vs. Wet Boiling Points
When brake fluid reaches its boiling point during operation, vapor forms in the brake lines, making it compressible. This produces a soft or spongy brake pedal that can undermine brake performance. If you frequently carve scenic mountain roads or rip through corners, the last thing you want is compromised stopping power.
Since brake fluid is hygroscopic, required industry testing simulates real-world field conditions. The wet and dry boiling points are tested to help illuminate the quality of a brake fluid.
Here is a quick description of boiling points:
Dry equilibrium reflux boiling point (ERBP): Measures the boiling point of the fresh fluid right out of the bottle before it has been contaminated by moisture.
Wet equilibrium reflux boiling point (WERBP): Measures the boiling point of fluid after it has had time to absorb moisture from its surroundings, including conditions under which the average vehicle operates (3.7% water contamination, to be exact).
Effects of Moisture Contamination
While brake fade caused by boiling fluid is an immediate concern, moisture contamination also has long-term consequences. Water in the brake system accelerates the corrosion of metal components like calipers, master cylinders and ABS control valves. Over time, this can lead to rust, leaks and expensive repairs.
AMSOIL DOT 3 & 4 Synthetic Brake Fluid
The unique chemistry behind AMSOIL synthetic brake fluids increases boiling points well beyond DOT standards, even when contaminated with 3.7% water. AMSOIL brake fluids maintain a stable viscosity over a wide temperature range and flow easily at the high pressures generated in today’s automotive brake systems to deliver solid, reliable braking. A nitrogen blanket is added to each bottle during manufacturing to purge moisture and ensure the maximum level of performance.
AMSOIL DOT 3 & 4 Synthetic Brake Fluid is overbuilt for extra safety and performance. With a wet boiling point of 368°F (187°C), it far exceeds the minimum requirements of 284°F (140°C) for DOT 3 fluid and 311°F (155°C) for
DOT 4 fluid.
In addition, its low-viscosity formula ensures excellent responsiveness in modern ABS and traction-control systems. AMSOIL DOT 3 & 4 Synthetic Brake Fluid maintains low compressibility under severe heat, delivering a steady and firm pedal feel. It protects calipers, lines and seals, helping extend the life of key brake system components.
AMSOIL DOMINATOR® DOT 4 Synthetic Racing Brake Fluid
AMSOIL DOMINATOR DOT 4 Synthetic Racing Brake Fluid provides racers with a brake fluid that delivers superior high-temperature performance, preventing brake fade and vapor lock. High boiling points of 580°F/304°C (dry) and 399°F/204°C (wet) minimize vapor formation. A nitrogen-blanketed design avoids moisture absorption during manufacturing and storage.
Operating conditions – and, thus, fluid life – vary depending on the environment, equipment type and application. As a rule of thumb, change your brake fluid every two years for maximum performance. However, if you’re driving involves spirited sessions or long descents, replacing your brake fluid annually is recommended to maintain peak performance. For racing applications, change AMSOIL DOMINATOR DOT 4 Synthetic Racing Brake Fluid once a year for maximum performance.
Don’t Push it Off
While changing brake fluid can feel like a chore you can delay, ignoring it invites moisture buildup, corrosion of key components and poor braking performance. Brake fluid is more than just another maintenance item; it’s your front line of defense for vehicle safety and performance. But with the right fluid and regular maintenance, your vehicle’s brake performance can remain as sharp as ever.
Video transcript: Has Your Brake Fluid Gone Bad?
Featuring John Gardner, ASE Master Certified technician, automotive technology instructor and host of Motorhead Garage, with AMSOIL Director of Product Marketing Len Groom.
Well, I have three containers of brake fluid here. If you had to put one in your car, which one would you choose? Welcome to this AMSOIL Tech Tip. Well, it’s pretty obvious right here.
I wouldn’t want this one. Yeah, this is old brake fluid. It’s been in a vehicle for a while and it’s kind of contaminated, but looks could be deceiving. These two here, well, they look pretty similar.
So we have to understand all about brake fluid. Brake fluid is hygroscopic. Hygroscopic means it actually absorbs water. So if you leave the cap off or you have a big container, it’s going to absorb water.
Well, that’s a problem because it’s going to lower the boiling point. We’ll look at that in a minute. But we need to check the brake fluid. I’m going to show you which one would you pick.
Why is that? Well, I actually have a little meter here that measures water content. So what I’m going to do is I’m going to stick that in this one here and it’s a little bit of yellow. It’s been out all night sitting here.
Now watch this. This one here, well, I got this out about a week. Bam. You see it go right to the red there.
There’s a ton of water content in that one. So that one’s going to boil. And when it boils, see, it makes air in the systems. Air is compressible and liquid’s not.
That’s not a good thing. When we talk about brake fluid, we have to understand the different types. There’s DOT 3, DOT 4, DOT 5, and DOT 5.1. I’ll make it pretty simple for you.
The higher the number, the higher the boiling point. Now there’s one exception. DOT 3, DOT 4, DOT 5.1 are all glycol based. DOT 5, well that’s silicone based.
So DOT 5’s for some special applications. Military vehicles, motorcycles, stuff like that. We don’t use that too often. But we do use DOT 3, DOT 4, and DOT 5.1.
The higher the number, the higher the boiling point. Use what your manufacturer service manual calls for but you can always supersede the number. You can go a little bit higher. For example, if my car called for DOT 4, I could put 5.1 in it.
Or if it called for DOT 3, I could put 4 in it. But I can’t go backwards. We talk about checking brake fluid. Now we can check it with that tool I just used a little bit earlier.
We can also use a refractometer and you just look through it. And when you look through it, exactly what we see right there is the content of what’s going on of the boiling of the brake fluid. We can also use these little test strips. We can put them down in there.
And this actually measures copper content, which is weird I know, but some of the nickel metal lines have some copper in there and the brake fluid starts to deteriorate it. We know it’s going bad. Now we actually boiled some brake fluid, which is pretty cool. You can see it right here.
It’s not a control test, but these are both DOT 4 brake fluids. And you can see the one on the left. It’s not boiling at all. The one on the right we left out overnight.
You can see the color change. It boiled pretty quick. Len, AMSOIL has us covered with brake fluid as well. We do.
We’ve got brake fluid for all of your different applications. Just, you know, check out your owner’s manual. Make sure you’re using what they recommend. But one of the things that I wanted to mention, especially in the climate that we work in, is once that brake fluid picks up some water, it freezes.
So your pedal will get extremely stiff in the cold. Something maybe people in the southern states don’t think of. But those of us that work in the north have those issues. So the main thing is use what’s recommended for the vehicle and change it.
And I love AMSOIL because theirs is specified for DOT 3 and 4. And it actually reaches the specifications of 5.1, which is awesome. Now brake fluid is serviceable, just like all the other fluids on your vehicle. For all your fluid service needs, go to amsoil.com.
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Alex Thompson is Director of Product Marketing at AMSOIL. He guides product strategy, technical positioning and market development for automotive and commercial lubricants. Thompson works with product-development and communications teams to connect product capabilities with customer and equipment requirements. His AMSOIL Magazine Tech Talk contributions address oil consumption, hydraulic-system operation and the role lubricants play in component protection and system performance.
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