Brake fluid reservoir cap indicating required fluid type; photo by Chris Chase. Click image to enlarge
By Jim Kerr
It is easy to see that one of the most important components on any car are its brakes, and one of the key components of any brake system is the brake fluid. Brake fluid is also one of the most-overlooked maintenance items on any vehicle. While many auto manufacturers don’t list brake fluid replacement on their maintenance schedules, this is a low cost maintenance item that, if changed every couple years, can extend the life of expensive brake components such as ABS and traction control units.
Brake fluids can be grouped into two categories: glycol-based and silicone-based. In Canada, under Transport Canada’s guidelines, all brake fluids must be classified in accordance with the American Department of Transportation (DOT) standards. When purchasing brake fluid, you will find it labelled as type DOT 3, DOT 4, DOT 5 or DOT 5.1. Each has a different boiling point and it is important to use the correct fluid for your brake system.
DOT 3, 4 and 5.1 are Glycol-based brake fluids. DOT 5 fluid is Silicone-based. Each type has advantages and disadvantages. Glycol-based brake fluids are made from various substances such as mineral oils, esters, glycol esters or synthetic oil. These fluids are hygroscopic, which means they will absorb water from the atmosphere. The more water that is introduced to the braking system, the lower the boiling point of the fluid, reducing its effectiveness. An advantage of glycol-based fluids is that they seal the brake system better than silicone-based fluids. Many vehicles will use DOT 3 brake fluid, which has a boiling point of 205 Celsius. On most vehicles, the type of brake fluid required is embossed on the brake master cylinder cap.
Some performance cars require DOT 4 fluid. With a boiling point of 230 degrees Celsius, the fluid can handle the additional heat of the brake rotors without creating bubbles in the brake fluid. Any bubbles in the fluid are compressible and will make the brake pedal feel very spongy and reduce braking capability.