Brake Fluid Basics

Brake fluid is often very underestimated in importance, especially choosing the correct brake fluid for a particular application. You might have heard people talking about brake fade and what they’re usually referring to, is the brake pedal going soft or completely going to the floor, combined with a drastic loss of braking force. Often drivers will think that their brake discs or pads are failing, when the truth is that the brake fluid is not good enough for the demanding driving conditions of the vehicle.

In this article we will attempt to demystify all those numbers on the brake fluid bottle and help you to understand some of the characteristics of brake fluid.

In choosing the best brake fluid for your application, you need to understand the DOT (Department of Transport) ratings. The United States Department of Transport (DOT) has devised several ratings for brake fluid, which are clearly labelled on the packaging and determine the fluid’s performance and wet and dry boiling points. A brake fluid must be categorized into one of the following DOT classifications: DOT 3, DOT 4 and DOT 5.1 are Glycol-based brake fluids and DOT 5 which is a silicon-based brake fluid. For OEM vehicles, in deciding which rating is best for your vehicle, always refer to your vehicle’s handbook.

Brake Fluid for Racing

Due to the extreme braking conditions in racing, your brakes can heat up significantly, leading to break fade in just a few laps. That’s because the brake fluid is boiling. Brake Fluid is a liquid that withstands compression and has a high boiling point. If a liquid starts to boil, it turns into a gas (steam) which can then be compressed. This is the main cause of a spongy brake pedal. DOT 3, DOT 4 and DOT 5.1 are hygroscopic and absorb moisture over time. Therefore, it is recommended to change your brake fluid regularly.

Brake fluid with a Dry Boil Point less than 280°C/360°F is suited for road use including DOT 5.1. Racing brake fluid should have a Dry Boiling Point of over 300°C/580°F and these are still mainly DOT 4 rated.

The more heat generated in your brakes the higher temperature rating your brake fluid needs to have. It is important to note that DOT 5.1 is a fast road brake fluid designed for longer service intervals, DOT 5 is a silicon-based fluid and should not be considered for race use.

Brake Fluid Boiling Point

Part of the standards that need to be met by the manufacturers of DOT fluids are the minimum dry and wet boiling points. Under racing conditions, where the braking is frequent and hard, the quicker the brake fluid will heat and once the brake fluid starts boiling, brake fade starts to occur.

What's the Difference between DOT3, DOT 4, DOT5 and DOT5.1 Brake Fluid?

DOT 3, 4 and DOT 5.1 are all glycol-based brake fluids and are controlled by standards set out by the Department of Transportation (DOT). The main difference between these 3 brake fluids is their boiling points. Part of the standards that need to be met by the manufacturers of DOT fluids are the minimum dry and wet boiling points. These are the minimum temperatures that the brake fluid must perform at before the brake fluid starts to boil, which can lead to complete brake failure. Since DOT 3, 4 and 5.1 are all glycol-based brake fluids they are compatible with each other, which means they can be readily mixed without harming your brake system. It is important to never mistake DOT 5.1 (glycol-based) with DOT 5 which is silicone-based and should never be mixed with any other DOT fluid.

The Importance of the Wet Boiling Point

It is important to understand what the Wet Boiling Point is and how it impacts your braking, especially in tough braking conditions.

What does ‘wet boiling point’ actually mean?

The ‘wet boiling point’ is simply a test conducted on brake fluid to determine how quickly it will decline over the course of its service life. The brake fluid is exposed to a humid atmosphere for a specified period, after which the boiling point is measured. Originally the test was a rough approximation of a brake fluid’s boiling point after two years installed in a vehicle, at which point the fluid would contain approximately 3.7% water by volume. The wet boiling point test was devised for fluid used in OE road vehicle applications, where the owner typically pays little attention to the brake system once the vehicle leaves the dealership.

In principle, brake fluid that has less of a propensity to absorb moisture, the higher the wet boiling point and the better the brake fluid will perform under extreme braking conditions.

Why do I need to change my brake fluid?

Brake fluid is hygroscopic, meaning that it absorbs moisture from the atmosphere. Even though the brake fluid is in a ‘sealed’ hydraulic system, water absorption still occurs over time. Water molecules slowly penetrate the brake hoses, caliper piston seals, etc. As the volume of water in the brake fluid increases, the fluid’s boiling point declines. Again however, the process of water infiltration is extremely slow, particularly with the materials and construction techniques used in modern brake components.

The most important brake fluid characteristics for track/racing applications

A High Dry Boiling Point is crucial in racing conditions. For racing or track cars that are frequently serviced, the brake fluid in the car should always be relatively fresh. Having a high dry boiling point means that the fluid has a high resistance to vapor lock, thereby preventing brake fade caused by overheating of the brake fluid.

Another important characteristic is low viscosity. The lower the viscosity, the more readily the fluid flows and as such, provides the most rapid actuation of all brake system components.

Lastly a good race brake fluid should have high lubricity which will reduce wear on parts.