Brake Fluid is one of the most important items in racing, but a product many do not understand. One of the first things you will find when you take a bone stock car to the track is that the brakes will begin to lose performance as a session gets longer or as you get faster and your braking gets heavier. This reduction in stopping power is known as brake fade. Many will first think to address mechanical brake fade as it is easier to understand and we get to buy cool new parts. We will cover mechanical fade in a future post, but the focus of our discussion here is on fluid fade which is somewhat less obvious and can be far more dangerous.
You are likely here for the product recommendations which you can find at the bottom of this post as well as in this Track Day and Racing Brake Fluid Comparison Spreadsheet which I will keep maintained on a quarterly basis.
- Why should I care about Brake Fluid?
- DOT3, DOT4, DOT5, DOT what?
- Wet Boiling Point
- Dry Boiling Point
- Which brake fluid is best for you?
- Containers: Metal Cans vs Plastic Bottles
- Brake Fluid Comparison Chart
If you ask an experienced racer or track day participant what the best brake fluid is, they will probably tell you Castrol SRF or Motul RBF600. In all likelihood, they asked someone the same question about fluid when they were starting out and were given the same answer. In this guide to Racing Brake Fluid I will go over the ins and outs of the purpose, the characteristics, and those cryptic numbers on the side of the bottle so you can make an informed decision as to what the best brake fluid is for your application. As an alternative to the aforementioned typical answers I recommend you look in to:
- Project Mu G-four 335 (Emotive or Amazon)
- Wilwood EXP 600 Plus (Emotive or Amazon)
- StopTech STR-660 Ultra Performance (Emotive or TireRack)
- Motul RBF660 (Emotive or Amazon)
- Endless RF-650 (Amazon)
Why should I care about brake fluid?
Before we dive in to the details we need to understand the importance of brake fluid. Brake fluid is possibly the most important "component" of your car. Brake fluid is a hydraulic fluid - this means it is a non-compressible fluid that transfers power. When your foot puts pressure on the brake pedal, this fluid runs through brake lines to each of the corners of your car. These lines are connected to your brake calipers applying pressure to the caliper pistons which in turn clamp your brake pads on to your brake rotors. The resulting friction between your brake pads and your brake rotors is what allows your car to stop.
The cornerstone of the hydraulic principle is that fluid cannot be compressed. When you release your foot from the brake pedal, the brake fluid should no longer be pushing against the brake calipers and the pedal should return to its original position. As you can imagine, if your brake fluid is no longer acting as a non-compressible fluid this can cause serious issues. How does this happen?
Brake fluid is hygroscopic - this means that brake fluid will absorb moisture when exposed to air. Though the brake lines run through a theoretically sealed system, brake fluid acts as a vacuum. It will absorb moisure through microscopic pores in brake lines and seals as well as the small vent in the brake fluid reservoir cap. This vent is necessary to allow air to displace the brake fluid as brake pads and rotors wear and more brake fluid is sitting in the calipers behind the pistons. After a year, brake fluid will typically have absorbed about 2% of its volume in water. After 18 months, that increases to about 3%. If you live in a wet or humid climate, these figures are likely higher. So what is the big deal?
The friction between your pads and rotors creates a significant amount of heat. During normal street use, brake rotors will typically see temperatures of about 200°C (392°F), but with track use it is common to see temperatures of 500°C (932°F) and beyond. Furthermore, as caliper and line temperatures heat up and then cool repeatedly, condensation occurs. With heavy track use you can expect an increase in condensation and for the resulting moisture to become trapped in your braking system more quickly than with regular street use.
Standard DOT3 brake fluid must have a minimum boiling point of (205°C) or (401°F) as measured when new. As brake fluid absorbs more moisture, the boiling point of the mixture in your brake lines gradually decreases. As braking components transfer heat to the fluid, it is possible that the fluid in your system is starting become gas which IS compressible. Brake fluid fade then occurs as your foot is pushing on both fluid and gas causing a spongy or soft brake pedal feel, or even worse, the braking system can experience vapor lock causing the pedal to go to the floor with no pressure being applied to the brakes at all.
What do DOT3, DOT4, DOT5, and DOT5.1 mean?
Hopefully you are thoroughly convinced of the importance of brake fluid and choosing the right fluid for your use case. I previously mentioned "standard DOT3 brake fluid", but what does that actually mean? The DOT in this case stands for the Department of Transportation. The Federal Motor Vehicle Safety Standard 116 regulates brake fluids in the United States and the FMVSS is administered by the US DOT's National Highway Traffic Safety Administration.
The FMVSS regulations specify three different performance requirements with specific boiling points and kinematic viscosities. Each of the grades DOT3, DOT4, and DOT5 map to these requirements. For any performance application but ice racing, we do not need to worry much about kinematic viscosities. These are specified to insure that your brake fluid does not freeze. For reference, the DOT specifies kinematic viscosities at -40°C and I hope you are not anywhere near these conditions.
|Dry Boiling Point
|Wet Boiling Point
|Kinematic viscosity at -40°C
Looking at DOT grades when shopping for brake fluid can often be confusing. In many cases the manufacturer will not have a data sheet and will not list the DOT grade. In other cases, a manufacturer will list a DOT rating such as DOT5.1 because it meets the dry boiling point and viscosity standards, but the wet boiling point will not meet the standard. DOT3 brake fluid is the most common type of brake fluid used in cars and trucks. DOT4 has become more common as the lower viscosity can help the responsiveness of anti-lock braking (ABS) and traction control systems.
To simplify things it is fairly safe to assume that the majority of performance brake fluid you are looking at is akin to DOT4 in viscosity. High performance brake fluid that we are considering for racing and track use far exceeds the minimum DOT4 and DOT5.1 standards for dry and wet boiling points and even though DOT5.1 standards are higher, you will often the performance of many manufacturer's DOT4 fluids to far surpass their DOT5.1 fluid.
That being said, it is said that the lower viscosity of DOT 5.1 rated formulas allows ABS systems to react even faster than DOT4 and makes bleeding brakes easier. The theoretical performance benefits include a difference in brake modulation and pedal retraction, but it is likely that mere mortals would not be able to tell the difference.
Why should you avoid DOT5 Brake Fluid in your modern performance car?
The chart above lists DOT5 and DOT5.1 but they have the same values. It is important to understand why DOT5 is not the same as DOT5.1. Whereas DOT3, DOT4, and DOT5.1 are glycol-based and can be mixed, DOT5 is silicone-based and has different properties. Remember when you read about that brake fluid is hygroscopic. This is only mostly true. DOT5 is not hygroscopic and resists moisture absorption. Weird. Whereas DOT3, DOT4, DOT5.1 are colorless or amber, DOT 5 is purple. Weird. It also does not eat through paint. Weird.
So what is wrong with this miracle purple fluid that is not harmful to paint finishes if you spill and resists moisture absorption? Your car likely came from the factory with DOT3 and every component of the system was designed with a glycol-based fluid in mind. If you want to switch to DOT5 the proper method involves changing every rubber or plastic component of your braking system to be DOT5 compatible. Not doing so can damage the seals.
The reason for new components is that mixing DOT5 with "standard" fluids the mixture may turn in to a messy gel. If you have ever flushed brake fluid before you will know it is hard to get every last bit of the old fluid out. This is not exactly what you want to deal with when flushing your system. Finally, since DOT5 does not absorb moisture it ends up trapping air resulting in foaming in aeration in regular driving. In some sense, having a hygroscopic fluid is an advantage in this case as those air pockets stuck inside DOT5 can cause your anti-lock braking system to malfunction. DOT5 is primarily used in classic cars, show cars, and pre-ABS Harley-Davidson motorcycles to prevent corrosion and is not recommended for your street car. If we could go back in time we would probably reverse the naming of DOT5 and DOT5.1, but this is the world we live in. Stay away from DOT5.
What is the wet boiling point?
We have talked a lot about hygroscopicity and its effect on brake fluid performance. This is important enough that all brake fluids must publish the boiling point of fluid that has absorbed water. The wet boiling point is the temperature of brake fluid at which it begins to form gas bubbles once it has reached 3.7% of water content by volume. The standards set for wet boiling point are a simulation of the characteristics of aged fluid. This scenario is quite likely for the neighbor's 10 year old family SUV that has brake fluid that could be mistaken for motor oil. This is not quite as relevant to those of us shopping for high-performance brake fluid swapped regularly in to our track day cars.
What is the dry boiling point?
Dry boiling point is the temperature of brake fluid at which it begins to form gas bubbles when it is fresh out of the bottle. Wet boiling point is a measure of extreme service life and modern braking systems prevent moisture absorption. On top of that, we know the importance of brake fluid and are choosing new fluid that we will change regularly. In turn, dry boiling point is a much more important characteristic than wet boiling point when choosing a brake fluid for track and racing applications.
As we have noted before, moisture draws its way in to braking systems through many avenues. It is important to always use a fresh bottle of fluid. If a bottle of fluid is open, you can assume it will have absorbed moisture from the air and will not perform at its specified dry boiling point. It is important to always use a fresh bottle of fluid.
It is also important to note that the drop from dry boiling point to wet boiling point is not linear. The chart above is included as a comparison of different DOT grade levels and their delta, not as a time series.
Which brake fluid is best for you?
The best brake fluid for you is one that meets the boiling point specifications for your specific application while matching your personal budget constraints. Here are some common considerations for choosing a brake fluid:
- the weight of the vehicle
- brake component temperatures
- length of time at high temperatures
- fluid service intervals
- the size of your wallet
If you have a light car that does not see tracks that see heavy braking, you might not choose a pricey fluid that has the highest outright dry boiling point. On the flip side, if you have a heavy car that easily boils fluid, you might want a high-temperature fluid even if you primarily run short track day sessions. Our shop Tesla Model 3 Performance is hardly an endurance racer, but we choose high-temp fluid with our stock brakes as we have heard of many who have been able to boil the fluid.
Your choice of fluid also depends on the frequency with which you bleed or flush your braking system. Many track day organization tech forms ask for fluid that is less than 6 months old. If you are a track junkie, your service intervals may be even shorter. The more frequently you change, the more you are taking advantage of the high dry boiling points. Brake fluid is a relatively low cost maintenance item given its service life. You are likely to spend many hundreds of dollars on pads and rotors and putting a good fluid behind that system is a relatively inexpensive path to peak performance.
Metal brake fluid cans vs. plastic brake fluid bottles
At the risk of not getting enough in the weeds, you may have wondered at some point why some brake fluid comes in metal cans and why some come in plastic bottles. As a middle-aged man I remember a time when brake fluid only came in metal cans. Metal cans are said to have a 25% longer shelf life than plastic bottles. It is common to find "boutique" brake fluid that comes in metal cans. As these are lower volume products it makes sense that manufacturers would use metal cans so the brake fluid can last longer on the shelf. The metal can itself will also start to rust if it has been contaminated by moisture letting you know that this is brake fluid you absolutely do not want to put in your vehicle. Practically speaking, you and I are buying fresh fluid (right?) so as long as we are purchasing from a reputable source metal vs. plastic is a moot point.
Brake Fluid Comparison Chart
The following is a summary comparison of 20 popular performance brake fluids, showing their boiling points and cost per liter sorted by highest dry boiling point. I, like many of you, am always on the hunt for value so I have included recently found prices. Though a liter is used as the standard unit as this is what many cars have capacity for, please note that some prices were taken by simply doubling the price of a 500ml bottle and does not account for volume discounts. In a future post I will cover what practical recommendations for brake fluid I would make and do a deeper comparison of commonly available fluids. Some of the fluids on the list seem like questionably sourced unicorn blood and have few reviews. Aside from the Castrol SRF and Motul stalwarts, I recommend looking in to Project Mu G-four 335, Wilwood EXP 600 Plus, StopTech STR-660 Ultra Performance Race, and Endless RF-650.
|Price per Liter
|Wilwood XR Race-Only
|AP Racing Radi-CAL R4
|Brembo HTC 64T Racing
|Project Mu G-four 335
|HKS Racing Pro Brake Fluid
|Wilwood EXP 600 Plus
|Ferodo Super Formula
|StopTech STR-660 Ultra Performance Race
|AP Racing Radi-CAL R3
|LIQUI MOLY RACING
|Hawk HP660 Hi Temp Race
|Brembo LCF600+ Racing
|StopTech STR-600 High Performance Street
|AP Racing Radi-CAL R2
All data for this chart and more fluids can be found on this Track Day and Racing Brake Fluid Comparison Spreadsheet so you can sort and make your own comparisons. Make a copy in your own Google Drive and you can make edits. Please email any changes or additions you would like to see to this post or to the spreadsheet. Happy motoring!