Brake Fluid - Street, Performance, & Racing

Brake fluid is a type of hydraulic fluid. The job of a hydraulic fluid is to apply force while under pressure. On a car or truck, this means brake fluid distributes the force needed to apply the brakes and bring the vehicle to a stop.

Brake Fluid Must Have The Following Properties:

• - Be incompressible
• - Have a high boiling point
• - Be non-corrosive
• - Maintain consistent viscosity (thickness).

Brake fluid must operate in a sealed system. It is hygroscopic, meaning it will absorb any moisture that enters the brake system. Contamination in brake fluid will reduce its effectiveness when stopping a vehicle.

How A Hydraulic Brake System Works:

1. The driver depresses the brake pedal.
2. The brake pedal moves a pushrod.
3. The pushrod applies force to the piston(s) in the master cylinder.
4. The pistons build up brake fluid pressure in the master cylinder and send it out through the brake lines.
5. The brake lines distribute the pressurized brake fluid to either a disc brake caliper or drum brake wheel cylinder at each wheel.
6. The piston in each brake caliper or wheel cylinder moves outward with pressure.
7. The force of the caliper piston squeezes brake pads against the brake rotor.
8. The force of the wheel cylinder piston pushes brake shoes against a brake drum.
9. The friction between the pads and the rotors (or shoes and drums) slows and stops the vehicle.

Brake fluid is commonly sold in 500ml (1.05 US pint) or 1L (1.06 US quart) bottles. Performing a complete brake flush requires a minimum of 1 Liter of brake fluid. This will provide plenty of fluid to bleed out the lines completely. If you suspect the old brake fluid is contaminated, more fluid may be needed.

DOT 3, DOT 4, and DOT 5.1 are the only types of brake fluid that can be mixed together. DOT 5.0 cannot be mixed with other brake fluids. All brake fluids are labeled with DOT (for the Department of Transportation) and a number that represents the boiling point. There are some differences in boiling point, viscosity (thickness), and the ability to absorb moisture. Using the wrong brake fluid may cause corrosion to brake lines and fittings, or reduce braking performance. Always use brake fluid that is compatible with your vehicle.

1. Make sure the vehicle is parked on a level surface.
2. Open the hood and locate the brake master cylinder reservoir. It is commonly mounted at the firewall (below the windshield), on the driver’s side. Check your owner's manual if you’re not sure. 
3. Check the brake fluid level. Most newer vehicles have a clear plastic reservoir that is marked with a "full" line. This allows you to check the fluid level without removing the cap. Older vehicles (80s and prior) may have a metal reservoir and cap. Remove the retaining clamp and cap to see the “full” line.
• - If the fluid level is low, add the correct brake fluid to the "full" line. Do not overfill.
• - Low brake fluid typically means the brake pads or brake shoes are worn. Sometimes it may indicate a leak somewhere in the brake system. 
• - If the brake fluid is too low and/or the brake light is on, the vehicle may not be safe to drive.
4. Replace reservoir cap and secure tightly.

What you'll need to have:

• - Sealed new bottles of brake fluid, usually (1) 1 Liter bottle or (2) 500ml bottles (Never use brake fluid that has already been opened, as it may have absorbed moisture.)
• - 5/16” and ⅜” (8mm and 10mm) boxed or brake bleeder wrenches
• - 3/16” or ¼” ID plastic tubing (about 18” long)
• - Clear plastic bottle or container
• - Floor jack and jack stands
• - A second person to depress the brake pedal while bleeding, or a power brake bleeder (optional, but highly recommended)

What you’ll need to do:

1. Make sure the vehicle is parked on a level surface.
2. Remove the cap from the brake fluid reservoir.
3. Make sure the brake fluid is filled to the “full” line.
4. If using a brake bleeder, connect and pressurize, per the instructions. Otherwise, have a person ready inside the vehicle
5. Start the farthest away from the master cylinder. Bleed the brakes in this order:
   • - Right Rear Wheel
   • - Left Rear Wheel
   • - Right Front Wheel
   • - Left Front Wheel
6. Lift and support the vehicle at each location.
7. Remove the wheel.
8. Pour a few inches of brake fluid into the clear plastic container.
9. Locate the bleeder screw on the brake caliper or wheel cylinder.
10. Remove the rubber cap from the bleeder valve.
11. Place the correct-size wrench on the bleeder valve.
12. Push one end of plastic tubing on the bleeder valve nipple.
13. Submerge the other end of the tubing in the container of brake fluid. This ensures no air enters the brake system when bleeding.
14. Have the other person pump the brake pedal a few times, until the pedal is firm.
    • - Make sure the brake pedal is held down while bleeding. 
    • - Lifting the brake pedal while the bleeder valve is open may cause air to enter the brake lines. 
15. Open the bleeder screw, letting the fluid flow into the container for a few seconds.
16. Close the bleeder valve.
17. Repeat steps 13 to 15 until:
    • - The brake fluid comes out a lighter color.
    • - There are no air bubbles in the brake fluid.
18. Make sure the bleeder valve is tight and reinstall the rubber cap.
19. Reinstall the wheel and torque lugnuts to specification.
20. Bleed the remaining brake calipers or wheel cylinders. Follow the order in step 3. Important: Top off the brake fluid reservoir to the “full” line before proceeding to the next wheel.
21. Once all four wheels are done:
    • - Fill the brake fluid reservoir to the “full” line.
    • - Pump the brake pedal until the pedal is firm.
    • - Test drive the vehicle.
    • - Repeat the brake bleed procedure if the brake pedal feels soft or spongy.

There are a few different types of brake fluid. The most common are DOT 3, DOT 4, and DOT 5. The higher the number, the higher the performance and boiling point. 

DOT 3

• -Glycol ether-based
• -Amber color 
• -Lowest boiling point (Boiling Point: 401 degrees Fahrenheit / Wet Boiling Point: 284 degrees Fahrenheit)
• -Hygroscopic, meaning it absorbs water
• -Needs to be replaced every few years to perform properly 

DOT 4

• -Used mostly by European automakers, but becoming more common in domestic vehicles
• -Glycol ether/borate ester-based
• -Translucent color
• -Higher boiling point than DOT 3
• -Performs better than DOT 3 fluids initially, but the boiling point degrades faster (Boiling Point: Minimum 446 degrees Fahrenheit / Wet Boiling Point: Minimum 311 degrees Fahrenheit)
• -Hygroscopic, meaning it absorbs water
• -Needs to be replaced every few years to perform properly
• -More expensive than DOT 3
• -Can be mixed with DOT 3
• -There are different subtypes of DOT 4 brake fluid: DOT 4 low viscosity - common on BMW / DOT 4 racing - often a blue color, higher boiling point / DOT 4+ - common on Mercedes-Benz and Volvo

DOT 5

• -Silicone-based
• -Purple color
• -Higher boiling point than DOT 3 or DOT 4 (Dry Boiling Point: 500 degrees Fahrenheit / Wet Boiling Point: 356 degrees Fahrenheit)
• -Hydrophobic - does not absorb water like the other types of brake fluid
• -Costs about the same as DOT 4
• -DOT 5 should never be mixed with other brake fluids

DOT 5.1

• -Designation can be confusing
• -Similar to DOT 4, but formulated to meet DOT 5 regulations
• -Glycol ether-based (not silicone like DOT 5)
• -Amber or Translucent color 
• -Boiling point similar to DOT 4 racing fluids (Dry Boiling Point: 500 degrees Fahrenheit / Wet Boiling Point: 356 degrees Fahrenheit)
• -Much more expensive than DOT 3 or DOT 4 brake fluid
• -Can be mixed with both DOT 3 and DOT 4 fluids.