Brake fluid is a type of hydraulic fluid that is used in hydraulic braking systems as well as hydraulic clutch applications, which can be found in almost every type of vehicle. It is able to convert force into pressure as it begins to amplify braking force. Brake fluid is able to perform this conversion by due to the fact that liquids are not considerably compressible.
Generally, most brake fluids used today are “glycol-ether” based. However, mineral oil, which is also known as “liquid hydraulic mineral”, or “LHM”, is available. Mineral oils are most commonly used in luxury vehicles such as Rolls Royce models. Silicone fluids, which are mostly found in military vehicles, are also available. As it stands, there are several different types of brake system fluids that can be categorized under three headings:
- Glycol-Ether based fluid
- Mineral oil-based fluid
- Silicone-based fluid
Categorizing Brake Fluids
The United States Department of Transportation classified brake fluid into four main categories:
- DOT 3
- DOT 4
- DOT 5
- DOT 5.1
“DOT” stands for “Department of Transportation”, and each category has its own unique dry and wet boiling point. Glycol-Ether based brake fluids include DOT 3, DOT 4 and DOT 5.1. DOT 5 is a silicon-based brake fluid.
While it’s true that the majority of cars will fall into the DOT 4 category, you should always check your vehicle’s manual for guidance and clarification. As stated earlier, each DOT category has its own unique boiling point. As each category increases, each boiling point for that category increases as well.
High Temperatures & Brake Fluid
In normal operation, brake fluid remains as a liquid and can’t be compressed, thus allowing the brake pads and shoes can be properly applied. However, under intense temperatures caused by heavy driving and the friction that follows, certain brake fluids inevitably hit their boiling point, thus turning it into a gas.
For example, after a few laps around a race track or when descending a long steep hill, the high temperatures created by the friction from the pads on the disc can cause the brake fluid to boil and it starts turning into a gas. This means it’s compressible, and suddenly the brake pedal begins to feel spongy. As a consequence, the braking efficiency is reduced and can fail altogether. Once the brake fluid cools down, the effectiveness will be restored.
That is exactly why it’s important to choose the correct brake fluid for all driving conditions. Each brake boiling point is as followed:
Wet Boiling Points
- DOT 3 – 284F (140C)
- DOT 4 – 311F (155C)
- DOT 5 – 356F (180C)
- DOT 5.1 – 356F (180C)
Dry Boiling Points
- DOT 3 – 401F (205C)
- DOT 4 – 446F (230C)
- DOT 5 – 500F (260C)
- DOT 5.1 – 500F (260C)
Always Stay Aware of the Risks
Glycol-Ether based brake fluid tends to have two significant disadvantages. First, it is hygroscopic, or in other words, it attracts water. Consequently, your vehicle’s braking efficiency will begin to reduce as a result of the fluid absorbing water. This is where most drivers fail to act. It is crucial for all vehicle owners to be aware of their brake fluid’s depreciation, which ultimately leads to the brakes “bleeding out”, thus requiring the brake fluid to be replaced.
Secondly, it is corrosive. You should always remain cautious as you replace your vehicle’s brake fluid, as the fluid will corrode any painted surfaces. Because DOT 5 mineral oil-based brake fluid and silicone-based fluid both have a wet boiling point of 364F (185C), it will not absorb water, nor will it harm any painted surfaces.
Silicone-based DOT 5 brake fluid has its own disadvantages and risks as well. It cannot be used in everyday road cars since it does not lubricate ABS pumps. In addition, DOT 5 fluids compress more than Glycol-Ether based brake fluids, resulting in a spongier brake pedal. Most drivers prefer a stiff brake pedal.
Lastly, never mix brake fluid types (even Glycol-Ether based ones) and always replace the lid on the bottle. However, we recommend buying fresh brake fluid whenever you top up or replace it in order to avoid possible water contamination. Learn how to check brake fluid and what to do if you see your abs light on.