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What is brake fluid?

Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in vehicles. It is responsible for transferring force into pressure, and to amplify 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 stops the vehicle. It works because liquids are incompressible.
 

DOT 3, DOT 4, DOT 5 and DOT 5.1 Brake Fluid. What’s the difference?

The Department of Transportation (DOT) classifies brake fluids to defined specifications. These specifications relate to their boiling points and chemical composition, both of which are important. All currently available brake fluids are covered by one of the following specifications; DOT 3, DOT 4, DOT 5 and DOT 5.1.

The DOT specifies two reference tests for brake fluids.

• Dry boiling point - the boiling point of fresh fluid

• Wet boiling point –the boiling point once the fluid has absorbed moisture (representing brake fluid after time spent in a real situation

  

DOT 3, 4 and 5.1 fluids are hygroscopic and glycol based, which means they absorb moisture from the atmosphere. Glycol based brake fluid starts to absorb moisture from the moment it is put in the hydraulic brake system or exposed to the air. The fluid attracts moisture through microscopic pores in rubber hoses, past seals, and exposure to the air. As water enters the system, instead of pooling in low spots (such as the calliper), due to its weight in comparison with brake fluid, it is dispersed throughout the whole of the brake fluid. This helps to keep the boiling point of the entire brake fluid high rather than having pools of water in the system which will boil much sooner than the rest of the brake fluid.

Also, the difference between DOT 3, 4 and 5.1 is the boiling point. The higher the boiling point, the more heat and abuse the fluid can take. Therefore, with regards to these 3 glycol based fluids, DOT 3 has the lowest boiling point and DOT 5.1 the highest. Most brake fluids used today are glycol based and are ideally suited for higher performance vehicles where the braking system routinely reaches moderate brake temperatures.

DOT 5 fluid is hydrophobic and silicone based, which means that it has the tendency to repel water. Silicone fluid does not absorb water from the surrounding atmosphere while in service and therefore offers greatly extended service life whilst improving corrosion resistance of the main components of the braking system. Silicone based DOT 5 was originally introduced to give higher temperature performance over glycol DOT 4, however, because it is silicone based, it does not lubricate ABS pumps as well as glycol based fluids.
 

The key take away here is that there are two types of brake fluids:

• DOT 3, DOT 4, and DOT 5.1 which are based on poly glycol compounds.
• DOT 5, which are based on Silicone.

Mixing glycol and silicone fluids is definitely not advised. Always check the spec of brake fluid required for your vehicle on the master cylinder or in the manufacturers handbook (your owner’s manual).