Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch application in vehicles. It is responsible for transferring force into pressure and amplifying braking force. Simply stated, when you apply your foot to the brake pedal, brake fluid transfers this force into pressure to the front and rear brakes and tops the vehicle. It works because liquids are incompressible.
Why is the Braking System and its Fluid Important?
The braking system on any vehicle is a critical safety system. It becomes even more critical in race and performance applications.
Bringing your car to a stop generates a lot of heat. The heavier the vehicle and the faster it is going, the more heat is created. The brake fluid you use needs to stand up to the heat and still do its job; transfer force from the brake pedal to the caliper or wheel cylinder.
The Important Factors Involved When Comparing Brake Fluids Are:
- Boiling Point
- Corrosion Prevention
What is it Made From?
Glycol-based fluids are hygroscopic, meaning they gradually absorb moisture from the atmosphere and disperse it throughout the system. As the water content of the fluid increases, its boiling point decreases. The additional moisture in the fluid will also start to corrode the metal components of the system.
The silicone-based fluid is hydrophobic, meaning it will not absorb moisture. If any moisture is introduced into the system, it can collect in pockets that can either freeze or boil off. This can lead to damage to the brake system and/or brake failure. Silicone is also more compressible than glycol, which can lead to a “spongy” feeling brake pedal.
Because of the way they deal with water, glycol and silicone-based fluids are NOT compatible with each other. Never mix the 2 types of fluid. Only add DOT 5 silicone brake fluid to a completely dry system or a vehicle that already has DOT 5 in it. Do not add anything other than DOT 5 to a system that calls for DOT 5 brake fluid.
Viscosity and Brake Fluids
Viscosity is a measure of the fluid’s “thickness.” For example, oil has a higher viscosity than water. Brake fluid must maintain its viscosity through both extreme heat and cold to provide reliable, safe braking. DOT 3 has the highest viscosity. As the DOT number increases, the viscosity goes down.
Corrosion Prevention and Brake Fluids
In addition to withstanding heat, the brake fluid must also resist corroding the metal brake system components. Additives are added to the fluid to accomplish this. The added chemicals protect the metal parts, but they will damage painted surfaces.
Glycol-based fluids are very harmful to paint. If the fluid is spilled or leaks onto a painted surface, the paint will be damaged. DOT 5 silicone-based fluid is much less harmful to paint.
Compressibility and Brake Fluids:
Brake fluid is intended to transfer force. If the fluid compresses, even a little bit, the force is reduced. Brake fluid must maintain a low level of compressibility to maintain the feel of the brake pedal and provide consistent performance.