HOW TO TEST YOUR BRAKE FLUID
Updated: Feb 21
Let’s be honest, you don’t really think about your brake fluid like you do other fluids in your car. Brake fluid isn’t as praised or talked about as oil or coolant, but it is just as important. Changing or flushing your brake fluid should be done at least once a year, and if you are on a track like us, it should be done once every or every other track day. We like to flush our brake fluid before every track weekend by running fresh fluid through the lines and purging out the old fluid.
But why is this so important? You can still stop so what’s the big deal? Lets first quickly talk about the different kinds of brake fluid and what some of the numbers mean on the back of the bottle.
Isn’t all Brake Fluid the Same?
I am sure you have walked down the aisle at your local parts store, or even scrolled through our fluids section on the Race German store. There are two types of brake fluid but only one is used in the automotive industry. 1. A petroleum based and 2. A non petroleum based. The non petroleum based is what we use, so let’s talk about the different kinds.
The most common fluids you will find are DOT 3, DOT 4, DOT 5, and DOT 5.1. DOT 3, 4, and 5.1 are all mixtures of glycols and glycol ethers, while DOT 5 is actually a silicone based fluid, not used in most modern cars or BMWs. DOT 5 is never to be mixed with DOT 3, 4, or 5.1 as they have a different kind of base fluid. DOT 3 has pretty much been replaced by DOT 4, which is what we will focus on, and by far the most popular in street and racing applications.
What is Boiling Point?
On the back of the bottle you will usually see a boiling point chart. The DOT gives minimums for manufacturers to meet to call their fluid DOT 3, or 4 etc. From there, manufacturers have their own recipes to increase these boiling points for higher heat race applications. The table will show a dry boiling point and a wet boiling point.
Before we talk about what these terms or numbers mean you first have to understand that glycol base brake fluids are hygroscopic. Webster defines hygroscopic as the tendency to absorb moisture in the air. Ah ha! So now you are starting to see why changing your brake fluid is so important. Glycol based brake fluids can actually absorb water in the fluid. If you can go back to your basic science class in middle school, you may remember that water is non compressible (or very hard to compress). The whole science behind brake fluid, is the ability for it to be compressed by your brake pedal and pushed through the car’s lines and into your calipers.
So let’s get back to those two terms that are on the back of the bottle.
Dry Boiling Point - The temperature at which the brake fluid boils before it absorbs moisture.
Wet Boiling Point - The temperature at which the brake fluid boils after it absorbs moisture.
You can see in the table below the boiling point temperatures for each grade of brake fluid. We all know that water boils at 212 degrees F, so once it starts to boil in the brake lines, it can produce steam. This steam is the gaseous version of water and is highly compressible compared to water. With the gas being compressed the brake system can no longer function like it was intended and you could experience your brake pedal go straight to the floor. This wouldn’t be the best time coming into turn 1 at 100 mph. You can see the wet boiling point is dramatically lower than the dry boiling point. This shows you how much water has an affect on brake fluid. (135 degree difference in DOT 4)
Replacing your fluid before track days is a highly recommended practice by us. So you may be asking, Ok, I understand all this by how do I know if my brake fluid is going bad? I am glad you asked that.
How to Test Your Brake Fluid
There are a few different methods to test the amount of water in your fluid, but the easiest is a brake fluid tester. This simple tool is a must have in any toolbox no matter if you are on track or not. It looks like a black marker, but when you remove the cap, it features two prongs or probes. The other end of the tester features LED lights that are green, yellow or red with a simple to read table to show the percentage of water that is absorbed into the fluid. To use, simply dip the two probes in your brake fluid reservoir and press the button on top. You will then see LED lights come on. Anything over 3.7% by volume is too much water absorption and you should replace your brake fluid. See the pictures below showing good and bad brake fluid.
I hope this quick overview of brake fluid shows you it is just as important as oil or coolant. Without good brakes you won’t be safe on the road or the track. I have actually experienced a car go off track because his brake fluid boiled and the pedal went straight to the floor. Another good thing to remember is once a brake fluid bottle is open, it starts to absorb moisture in the air from humidity. If you open a brake fluid bottle and use half and decide to place it on the shelf for another date, it most likely will go bad by the next time you want to use it again. It is good practice to use the brake fluid tester in open bottles before pouring the fluid into your car.
We carry a wide array of brake fluids and the brake fluid tester. If you have any questions, please don’t hesitate to reach out to us at firstname.lastname@example.org
Race on and be safe.
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