Brake Fluid Notes and Information

I read an interesting article on brake fluid recently that got me intrigued on the subject to dive into it a bit more and see what all I could learn about it. This article by Mike Mavrigian of Birchwood Automotive stated that different brake fluids have different compressibility. He stated, "Different brake fluids can dramatically alter the brake pedal feel with no other system changes." This all being based on the different compressibility of the various brake fluids. This opened my eyes a bit. In our race cars we have predominantly used Motul RBF600 and we have never had any issues or concerns so why fix something that isn't broken. But if we can change the brake pedal feel simply by changing brands of brake fluid perhaps that is something that we should look into.

In particular, the Honda platforms, although having great stopping power, seem to have a softer pedal feel. At the very basic level the first thing people look at when it comes to racing brake fluids are the dry and wet boiling points. Caution, fancy word time -- Hygroscopic. Brake fluid is Hygroscopic, meaning it absorbs water. The dry boiling point is the threshold where the brake fluid will boil as it is brand new, fresh out of its container. The wet boiling point is the threshold where the brake fluid will boil once it has absorbed moisture. From the moment a brake fluid container is opened it begins absorbing moisture. Whether it is sitting on a shelf or being used in the braking system of your car. This is why it is important, especially in track / race cars to bleed and change brake fluid on a regular basis. No one wants to see what it is like to go into a high speed braking zone with no brakes.

This information is predominantly focused on track use cars but the information carries over into the street car world as well. Porsche is about the only company I have seen that actually has a 2 year change interval specified in its maintenance intervals for their produced street cars and is adamant about making sure the customers know. But that doesn't just hold true to their vehicles. You should have a regularly scheduled complete brake fluid flush in your vehicles maintenance schedule.

The 3 main grades of brake fluid are DOT 3, DOT 4 and DOT 5.1. All of those options are mixtures of glycols and glycol ethers and are capable of being mixed with each other if need be. Keep in mind mixing a DOT 3 and a DOT 4 fluid will bring the Dry and Wet boiling points to the lower level fluid.

DOT 3 Brake Fluid: Minimum Dry Boiling Point Standard – 401° Minimum Wet Boiling Point Standard – 284°

DOT 4 Brake Fluid: Minimum Dry Boiling Point Standard – 446° Minimum Wet Boiling Point Standard – 311°

DOT 5.1 Brake Fluid: Minimum Dry Boiling Point Standard – 500° Minimum Wet Boiling Point Standard – 356°

With the boring stuff out of the way what does this mean to you? If you track your car you generate a lot of heat in the brake system. Boiling brake fluid is not a fun adventure when it comes time to use your brake pedal. Although the dry boiling point is an important number to look at, the available racing brake fluids on the market all have a sufficient dry boiling point for track use. The more important number is the wet boiling point, the point your brake fluid will boil once it has moisture introduced into the system. Moisture into the system can happen almost immediately once putting the brake fluid into your system. It doesn't sound like a lot but as little as 1-2% moisture absorption can dramatically affect the braking system. This is why it becomes important with your track car to regularly bleed and flush your brake fluid. Bleeding is a quick way to rid the system of any air bubbles that may have arrived in the system and is generally done within the week prior to having on track brake use. Air bubbles can form when the excess moisture in the system boils and creates steam. The steam separates from the water and creates air pockets in your brake lines.

A regularly scheduled full brake flush is another part of keeping your brake system in track ready condition. A full flush on a yearly basis seems to be a good choice with regularly system bleedings throughout the year.

So what brake fluid is best for you? Here is a chart of some of the dry and wet boiling points of some of the most popular brake fluids on the market. The Castrol SRF has a strong argument here. Pricey but check out that wet boiling point threshold. The Castrol SRF is a popular option for track drivers that are using their daily driven street cars. They can do a full flush on a yearly basis and in general may not have to do as many interval system bleeds as other brands. ° in Fahrenheit

But what does all of this do in regards to the original inquiry as to why I wanted to investigate brake fluid - Compressibility? Well unfortunately compressibility is not an official spec that has to be listed by brake fluid manufacturers so it comes down to good ole trial and error. The compressibility is based on what is known as air solubility. That rating can be as high as 7% dissolved air in the brake fluid in brand new, sealed condition. That is ultimately what will affect the pedal feel of the various brands of brake fluid. Trial and error in each application will be the only way to realize any affect on pedal feel by changing from one brand of brake fluid to another. I will quote Torque Racing Brake Fluid in regards to this report: "A research report published by Union Carbide demonstrates a relationship between the compressibility of a brake fluid and its density (or specific gravity). The greater the density of a brake fluid the less compressible it is. As there is no DOT specification, within the scope of the Polyalkylene Glycol Ether-based fluids there can be a density difference range over 200% between fluids!" Brake Fluid

Tips:

- Keep master cylinder cap sealed as much as possible to avoid moisture entering the system.

- Bleed brake fluid as needed to rid the system of air bubbles.

- Flush brake system in your track used car on a yearly basis.

- Pay attention to those wet boiling point numbers.

- Pedal feel may alter dramatically from brake fluid to brake fluid so don't feel bashful about trying other brands.

- Do not store opened containers of brake fluid for long periods of time.

Sources:

- The Shop, May 2016, Brake Fluid Primer by Mike Mavrigian

- Torque Racing Brake Fluid - http://torquebrakefluid.com/abc.html

- Tilton Racing - http://tiltonracing.com/brake-fluid-stay-safe-and-go-fast/

- Leland West - https://www.lelandwest.com/brake-fluid-comparison-chart.cfm

- Carbotech - http://www.ctbrakes.com/brake-fluid.asp