Brake Fluid Types and Differences
Brake fluid has gone through several evolutions in the last ten or so years with new formulas, materials, and standards being introduced. I swept away the clutter and this page should help you understand each type of fluid and why it exists.
First, the golden rule of brake fluid: change it often. And change it more often if it's a track car. Nearly all brake fluid on the market is hygroscopic and will absorb any moisture. The higher water content, the thicker the fluid, and the more likely it is to boil under extreme heat. So it's vital that you change your fluid regularly. Some brands require fluid changes more often than others so pay attention to the advice on the label. No brake fluid should be in use for longer than two years. Once opened the shelf life of a can is generally considered to be no more than one year.
The key to a good brake fluid is how much it can resist compression when pressure is applied. The more the fluid compresses the longer the pedal travel and the longer it takes for the fluid to push on the caliper pistons. Fresh brake fluid out of the bottle is very resistant to compression but as it absorbs moisture and/or air it will compress more and more (water and air have very poor resistance to compression).
All brake fluid can be divided into two types: glycol hygroscopic and silicone hydrophobic.
GLYCOL-BASED WATER ABSORBING (HYGROSCOPIC). The vast majority of brake fluid will absorb moisture to keep it from interfering with the braking system. Not only does water compress a lot but it can also rust or even freeze in the internal metal brake lines and calipers. And it's a poor lubricant for sliding caliper pistons. Also, as water boils it creates a gas that can block fluid from circulating. This is the number one reason why brake fluid should be changed often (at least every two years for a normal street car). The boiling point of brake fluid is used as the metric for evaluating its performance and the US DOT has issued grades based on the fluid's boiling point. The higher the grade, the higher its boiling point. Fresh from the bottle fluid is considered "dry" with its highest boiling point. All glycol fluids can be mixed and combined but realize the fluid is only as strong as its weakest rating. That's why if you change the fluid often you are always guaranteed to have the highest quality in operation. Glycol and full silicone fluids should not be mixed (see below).
|Grade||Dry Boiling Point
(no moisture content)
|Wet Boiling Point
(moisture after one year)
|DOT3||205° C (401° F)||140° C (284° F)||normal|
|DOT4||230° C (446° F)||155° C (311° F)||normal|
|DOT5*||260° C (500° F)||180° C (356° F)||normal|
|DOT5.1*||260° C (500° F)||180° C (356° F)||low|
* There is a lot of confusion on the DOT5 rating. Strictly speaking the DOT5 is the current highest wet boiling point. But the first product that reached this spec was a full silicone-based hydrophobic fluid and the DOT5 label somehow became synonymous with only silicone fluids. Advances in the chemical composition of glycol-based fluid has resulted in some products exceeding minimum DOT5 ratings. But to avoid confusion or misrepresentation, manufacturers choose to label these as DOT4 or "exceeds DOT4 specifcations". The DOT5.1 rating is a low viscosity, non-silicone glycol-based fluids that meets the DOT5 boiling points. However, you could have a low viscosity fluid that does not meet DOT5 minimum boiling points (Ate SL6 for example) and these continue to be a DOT4 rated fluid. Don't be surprised if the DOT issues a new rating system in the future to hopefully clear this up.
DOT3 - BMW adopted DOT4 fluid in the early 1990s so we're not going to concern ourselves with DOT3 much. Any DOT4 is chemically compatible with a DOT3 system.
DOT4 - the most commonly found performance brake fluid. BMW was using DOT4 since the 1990s so this is what is considered the absolute minimum rating for optimum street braking performance. A DOT4 fluid is usually the minimum required for track use because they are a normal viscosity (not low) but remember to bleed and change the fluid often. Some DOT4 brands meet or surpass DOT5 boiling point specs but the manufacturer chooses to label them as DOT4 or "excceds DOT4 specs" (see above).
Products with DOT4 rating: Ate Type 200, Ate Super Blue**, Motul RBF600, Motul RBF660, Red Line RL-600, Genuine BMW Low Viscosity
DOT5.1 - a low viscosity glycol-based fluid. In the BMW world (and likely others), a change was made in the mid 2000s to a low-viscosity fluid (E53 chassis from 2004 and every new chassis from the E60 onwards). The low viscosity means a thinner fluid that can better travel through the tiny channels in the ABS and DSC systems. A thicker fluid will have more trouble moving through these passages and will be less effective. In most situations the viscosity goes unnoticed and you can use any glycol fluid you wish, especially on the track where higher temperatures will thin the fluid anyway. However, in cold climates the thinner low viscosity fluid will work better. If you experience a mushy or slow reacting pedal during an ABS stop on the street, you likely have the wrong fluid and changing to a low viscosity (LV) fluid will work much better. Note that LV brake fluid is generally not recommended for track use as the viscosity is too thin at higher temps. What works best is using a high performance or racing normal fluid which will thin to a lower viscosity on the track. These racing fluids typically exceed DOT5 boiling points and resist compression better than the LV fluids anyway.
Products with DOT5.1 rating: Motul DOT5.1, Pentosin Super DOT 5.1
Ate SL.6 is listed as a DOT4 even though it is low viscosity but it's wet boiling point does not meet DOT5 specs
** A note about color... In the US, all brake fluid must be an amber/yellow color. This is to distinguish it from other fluids (engine oil = brown, transmission/power steering fluid = red). So all DOT-approved brake fluids are going to be the yellow/gold color. And that was the only reason why Ate Super Blue was discontinued for good in 2013. Despite being sold as a 'racing' brake fluid and not a product for street use, Ate pulled the product from shelves (there was no issue with its formula or performance). Ate Type 200 is the exact same spec as the Super Blue and is the correct amber/yellow/gold color.
SILICONE-BASED WATER REPELLING ("DOT5") (HYDROPHOBIC). Silicone fluid makes for an outstanding racing brake fluid. Why? Because it repels water. Unlike glycol fluids that absorb water, a silicone fluid (greater than 70% content) will not mix with water. Since it never gets contaminated by water it has a much higher boiling point than a hygroscopic fluid. However, since the water is no longer absorbed it tends to pool and then boil on its own (at 212° F) and turn into vapor. The vapor blocks the system and can do more harm than absorbed water. The trapped air can give a very spongy pedal since it compresses easily. And pooled water will lead to rust. So silicone fluids will need to be bled far more often than glycol fluids to vent the trapped air and purge the water. Because of a misunderstanding when silicone was first introduced, the "DOT5" label seems to only apply to silicone-based fluids. This has resulted in even more confusion when the DOT5.1 came out (see above). A silicone fluid can be used in an existing brake system that previously had glycol fluid but the complete system must be drained and several flushes with the silicone fluid done to ensure no trace of the old fluid remains. The better approach would be to start fresh with all new components.
Products with "DOT5" rating: Castrol SRF