Brake Fluid 101

Posted by Dan on 6th Mar 2024

Brake Fluid 101

Why do cars have brake fluid?

 All cars have brake fluid.  A car needs brake fluid in order to apply pressure to your brake pads and ultimately your rotors to stop your car.  Brake fluid is incompressible where air is compressible.  A brake system with air in it will not provide a solid pedal and will not perform well.  Brake fluid hygroscopic which means it attracts water.  Water is also not something that you want in your brake system.  Water boils at a lower temperature and a boiling fluid will introduce air into the system.  Water boils at 212 degrees Fahrenheit wouldn't it work as brake fluid as long as it doesn't boil? Unfortunately that low of a boiling point would not work for long because it will almost certainly boil.  Brake fluid also has to be anti-corrosive so water/moisture is also not advantageous on that front.  A good brake fluid should also have a low viscosity.  A low viscosity will allow the fluid to flow freely and a low viscosity is also better for ABS.

Dry Boiling Point:

When you are looking at a brake fluid there should be a dry boiling point listed.  This dry boiling point refers to when the fluid will boil when there is next to no moisture in it.  In other words this is the boiling point of brand new fluid.  You might think the higher the dry boiling point the better right?  Not necessarily, you also want a good wet boiling point.

Wet Boiling Point:

The wet boiling point is when there is moisture in the fluid due to brake fluid being hydroscopic.  For a DOT4 fluid a wet point is when the fluid is 3.7% water of its volume.  That doesn't sound like much right because it is not.  Most fluid in your average passenger cars are wet.  So for a passenger car you can see how a high wet boiling point may be more important that a high dry boiling point.

Types of Brake Fluid:

There are 4 types of brake fluid; DOT3, DOT4, DOT5, and DOT5.1.  DOT3, 4, and 5.1 are glycol based and can be mixed.  DOT5 is Silicone based and can never be mixed with the other brake fluids.  DOT5 brake fluid is not widely used because bubbles can form easily in the fluid when it gets hot and are very difficult to get out.  DOT 5 fluid does not absorb water so it is good for extremely long periods of time.  However water that gets into the system will separate and cause localized corrosion.  

DOT3, 4, and 5.1:

These fluids are similar.  The key thing to know is that the dry and wet boiling points increase as the grades increase.  DOT 3 is has a standard dry boiling point of 401 degrees F and a wet boiling point of 284 degrees F.  DOT 4 446 degrees F dry and 311 degrees F wet.  DOT 5.1 500 degrees F dry and 356 degrees wet.  DOT 3 and 4 are best suited for passenger vehicles where DOT5.1 is best for higher end performance cars or racing applications.  Now these ratings are just DOT standards brake fluids can be made not to a DOT standard.  For example there are racing brake fluids that do not have a DOT rating.  

Testing Brake Fluid:

There are two ways you can test your brake fluid and both of which are aspects you should be concerned with.  First you can use a moisture content tester. You can find these on amazon for rather cheap ~$15 and they will tell you if you have dry or wet fluid and you can make the decision of when to replace it. The second type of test is to tell you the age of your fluid.  A fluid can be old and be in need of replacement and not be at its wet point yet.  This test can be done using test strips that can also be found on amazon.  

Selecting a Brake Fluid:

So which brake fluid do you use? It depends on the application. If you are racing you probably want a high dry boiling point and can sacrifice wet boiling point and accept that you will need to change fluid more often. Perhaps you want just a street car and you can accept a lower dry boiling point for a good wet boiling point. Below is a good resource for comparing brake fluids. At GR Performance Parts we like to use Motul RBF600 as it has a good wet and dry boiling point but selection is up to the user and the application.