Car servicing: How brake fluid works

Brake fluid is a specialised hydraulic fluid that operates within the braking system of your vehicle. It operates under extreme conditions, such as high temperatures and pressure, making it an essential component in the braking system.  Its primary function is to transfer the force applied to the brake pedal into pressure at the brake calipers or wheel cylinders, ultimately leading to the friction required to slow down or stop your vehicle. A strong braking system is paramount to keeping you safe as you navigate the roads of Britain and beyond and brake fluid is a vital part of that.

In this blog post we will take a look at what brake fluid does and where it fits within the braking system as a whole.

Components of the Brake System:    

1. The Master Cylinder: At the core of the braking system is the master cylinder. When the brake pedal is depressed, the master cylinder generates hydraulic pressure, which is then transmitted through the brake lines.
2. Brake Lines: These are a network of pipes that carry the pressurized brake fluid from the master cylinder to the brake calipers and wheel cylinders located at each wheel. 
3. Brake fluid:  A Hydraulic fluid used to transfer braking pressure from pedal to pad
4. Brake Calipers or Wheel Cylinders: Brake calipers are found in disc brake systems which are used by most modern cars, while wheel cylinders are part of drum brake systems. Both components use the hydraulic pressure from the brake fluid to force brake pads or shoes against the brake rotors or drums, creating the friction necessary to slow down the vehicle.

How the brake system works: 

1. Compression and Pressure: When the driver applies the brake pedal, the master cylinder compresses a piston. This action pressurises the brake fluid within the master cylinder.  
2. Transmission of Pressure: The pressurised brake fluid is then transmitted through the brake lines. Brake lines are designed to withstand high pressure to ensure the force is efficiently delivered to the braking components at each wheel.   
3. Actuation of  Brake Calipers or Wheel Cylinders: In disc brake systems, the pressurised brake fluid enters the brake calipers, causing pistons to push the brake pads against the brake rotors. In drum brake systems, the fluid activates wheel cylinders, forcing brake shoes against the brake drums.
4. Friction and Deceleration: The resulting friction between the brake pads and rotors or brake shoes and drums generates friction, slowing down the wheels and, consequently, the entire vehicle.
5. Return to the Master Cylinder: Once the brake pedal is released, the pressure in the system decreases, and the brake fluid returns to the master cylinder, ready for the next braking event.

Types of Brake Fluid:

There are various types of brake fluid with the following being the most common in consumer cars. The boiling point of any brake fluid is measured by its DOT rating which is simply an abbreviation for Department of Transport.  the higher the DOT rating the higher the boiling point of the fluid.

DOT 4:

• Offers a good boiling point for road use making it suitable for most modern vehicles.
• Less prone to moisture absorption, providing enhanced performance and longevity than older DOT 3 fluid.
• Compatible with DOT 3 systems, making it a popular upgrade for older vehicles.

DOT 5.1:

• Possesses a similar chemical composition to DOT 3 and DOT 4 but with a higher boiling point.
• Suitable for high-performance and heavy-duty applications.
• Compatible with other glycol-based brake fluids.

DOT 5:

• Silicone-based fluid with a higher boiling point than glycol-based fluids.
• Not compatible with glycol-based fluids, requiring a complete system flush before use.
• Resistant to moisture absorption and corrosion, making it an ideal choice for classic cars and show vehicles.

Functions of Brake Fluid:

Brake fluid serves several critical functions in a vehicle's braking system:

• Transfer of Force: Brake fluid converts the force applied to the brake pedal into hydraulic pressure and Transmits this pressure to the brake calipers or wheel cylinders, initiating the braking process.
• Heat Dissipation:  Brake fluid absorbs and dissipates the heat generated during braking to prevent the braking system from overheating. The boiling point of the brake fluid is crucial in determining its ability to withstand high temperatures. Which is why performance vehicles use fluid with a higher DOT rating.
• Corrosion Prevention: Brake fluid protects the various components of the braking system from corrosion and wear.

Importance of Brake Fluid Maintenance:

Brake fluid plays a critical role in maintaining the effectiveness of your vehicle's braking system. Over time, brake fluid can absorb moisture, which can lead to a decrease in it's boiling point which compromises it's ability to transmit hydraulic pressure effectively. Regular inspection, maintenance and timely replacement of your brake fluid are essential to ensure optimal brake performance and, more importantly, your safety on the road.

Here are a few benefits to regular maintenance of your brake fluid

• Boiling Point Preservation: Regular maintenance prevents the brake fluid from reaching its boiling point, which could lead to brake fade and a loss of stopping power.
• Moisture Control: Moisture absorption over time can compromise the fluid's performance. Regular checks and replacement help maintain the fluid's integrity.
• Component Longevity: Properly maintained brake fluid contributes to the longevity of brake system components, reducing the risk of expensive repairs

We recommend changing the fluid in your braking system at least once every two years which is why we often suggest this alongside your minor service, major service or MOT test.