It goes without saying that a vehicle’s braking system is a critical part of driving. With that in mind, knowing how to best maintain it is key. So, when it comes to brake fluid, understanding the DOT ratings available can help to choose the right one for your vehicle, so that you can maintain the brakes and keep them in good working order. In this blog, we’ll take a closer look at the DOT ratings and why they exist.
What is brake fluid used for?
Brake fluid ensures that the car can stop effectively, by transferring brake pedal force to the brake pads. It’s known as a type of hydraulic fluid as it is responsible for moving parts within the braking system.
Brake fluid transmits hydraulic pressure to the brake calipers or wheel cylinders, which then press the brake pads against the rotors.
You can find out everything you need to know about brake fluid here.
Most importantly, the brake system of your vehicle works at high operating temperatures. This means that brake fluid needs to meet certain criteria in order to work effectively, with factors such as low viscosity, lubricity, corrosion resistance and a high boiling point. This is where DOT ratings come in.
What is a DOT rating for brake fluid?
DOT stands for Department of Transportation, who set the ratings. The DOT rating indicates the fluid’s performance, providing an understanding of the fluid’s resistance to heat and moisture.
In particular, the DOT rating indicates the fluid’s dry and wet boiling points. The dry boiling point is the boiling temperature of new, uncontaminated brake fluid (with no absorbed moisture).
On the other hand, the wet boiling point is measured after the fluid has absorbed a specified amount of moisture (3.7% water by volume).
The most common ratings in use today are DOT 3, DOT 4, DOT 5 and DOT 5.1.
What’s the difference between the DOT ratings?
The biggest difference between all the DOT ratings are the minimum boiling points, which means they are suitable for different cars or driving styles. Typically, the lower the DOT rating, the lower the boiling points of the brake fluid.
Secondly, different ratings can mean they are either glycol or silicone based. DOT 3, DOT 4 and DOT 5.1 are all glycol-based. This means they are chemically compatible but mixing is generally not recommended, as it reduces performance to the lowest specification present.
DOT 5 brake fluid is silicone-based and generally not recommended for ABS because it can aerate and trap air, affecting performance.
Which DOT rating brake fluid should I use?
It’s always important to check your vehicle’s handbook as this will typically clarify which brake fluid is most suitable for your vehicle. However, it’s worth noting that generally DOT 5 brake fluid is primarily used for the likes of classic cars or motorcycles, while DOT 5.1 brake fluid is popular choice for performance vehicles. Most everyday vehicles will require DOT 3 or DOT 4 brake fluid.
At Prestone, our DOT 4 brake fluid performs in extreme temperatures, with a dry boiling point of 260°C and a wet boiling point of 150°C. It’s ideal for use in all braking systems, providing the necessary corrosion protection.
