In the United States, the U.S. Department of Transportation (DOT) has created four different classes of brake fluid:
- DOT 3
- DOT 4
- DOT 5
- DOT 5.1
So what’s the difference you ask? It more or less comes down to their respective boiling points.
Brake fluid boiling point comparison chart
|Dry Boiling Point||Wet Boiling Point||Composition|
|DOT 3||205 C / 401 F||140 C / 284 F||Glycol ether|
|DOT 4||230 C / 446 F||155 C / 311 F||Glycol ether / borate ester|
|DOT 5||260 C / 500 F||180 C / 356 F||Silicone|
|DOT 5.1||260 C / 500 F||180 C / 356 F||Glycol ether / borate ester|
The specifications also set a number of other factors such as kinematic viscosities, pH values, high-temperature stability, chemical stability, corrosion, water tolerance, compatibility (sludging, sedimentation, and crystallization) and resistance to oxidation, but for the purposes of understanding the basics of DOT grade differences, these are less relevant.
What is DOT 5 brake fluid?
You may have noticed that there’s an outlier in the above chart. DOT 5 fluid is a silicone fluid, meaning it is not based on petroleum. This has the advantage that a DOT 5 fluid will not absorb water (more on that in a minute), which seems great, so why doesn’t everyone just use it? DOT 5 fluid has a few downsides:
- Incompatible with all other fluids so any system needs to be flushed thoroughly to run DOT 5
- By not absorbing water, any water that enters the system will sink to the bottom and start to rust there
- Pouring silicon fluid tends to cause air bubbles which creates a spongy feel in the brake pedal
- Because of their greater viscosity, DOT 5 fluids are typically incompatible with ABS systems. The pulsing of the system can cause air bubbles to coalesce , damage the valving and reducing braking capability.
- Most importantly: silicone fluids like DOT 5compress more than glycol fluids and are therefore not recommended for racing
The recommended use for DOT 5 fluid therefore is classic cars which are driven infrequently. By absorbing no moisture, these cars can be ready to go at any moment’s notice.
Ok, then what’s the difference between wet and dry boiling points?
Glycol fluids like DOT 3, DOT 4 and DOT 5.1 are actually designed to absorb water instead of separating like DOT 5. This water absorption means glycol-based fluids are “hygroscopic”. Once a fluid starts to absorb water, its characteristics and performance will change, which is why all brake fluids always list their so-called dry and wet boiling points.
Moisture can enter a braking system in a number of ways. For example, when you remove the reservoir cap to add fluid, when seals in the system start to wear or even through some rubber brake lines themselves. The wet boiling point therefore is a more accurate measure of the performance of a braking fluid.
A dry brake fluid is a brand new one under perfect conditions with minimal water content. It’s basically the fluid as it comes out of the bottle. Once a fluid goes into use it will start to absorb some amount of water and that will lower the boiling point. That’s simply because water boils at 100 C, thus lowering the boiling point of the brake fluid + water mixture. The wet boiling point therefore is the measured boiling point at or near the end of a brake fluid’s usable life, defined as having 3.7% water content.
Because of the hygroscopic nature of glycol fluids, it’s also not advisable to leave brake fluid open to the elements or to use stored open bottles of brake fluid.
What is “brake fade”?
Brake fluid is placed under intense pressure when braking. Greater pressure increases heat which can potentially causing the fluid to boil. Boiled fluid will cause brake fluid gas in the brake lines, which is compressible, and that leads to a soft pedal. This is known as “brake fade” and it’s something drivers actively want to avoid.
It’s typically not an issue on street cars, but it’s something that’s actively managed in race cars and can certainly happen if you take your car to a track day. If you’ve ever experienced it, it can be mild and semi-manageable or it can be near-catastrophic and feel like you have no brakes. This sucks and it’s for sure scary. To drive as effectively and safely as possible, you have to be confident that your brakes will perform on lap 10 as they did on lap one, so make sure you choose the right fluid.
There are other potential sources of brake fade as well. When a brake pad is pushed into the rotor, the pads release gasses which reduce the contact area between the two surfaces. Many high performance and racing rotors are therefore slotted and/or drilled to help these gasses escape and limit fade.
So what brake fluid then?
There’s no “right” answer here necessarily and DOT ratings are only one part of the equation. In many cases, as brake fluid will actually exceed its DOT rating, so we encourage everyone to look into the details of their specific fluid. As with most products, increasing performance increases the price, so make the choice that’s right for your application and budget. Be aware that it’s good practice to flush the system every couple of years and typically more often for race cars.
Click here to see the brake fluids we carry at Torqued.