Hydraulic brake systems rely on an incompressible fluid to transmit force. Because of the temperatures generated from the friction between the pads and rotors, brake fluids were specifically developed to work in this hostile environment. There are two types of brake fluids: ethylene glycol-based (DOT 3, 4 and 5.1) and silicone (DOT 5). Glycol brake fluids are the most prevalent and recommended by virtually every automotive manufacture - but they are also hygroscopic. This means that they absorb moisture from the air, which will eventually contaminate the brake fluid and ultimately lower the boiling point and prematurely wear rubber components. There are many ways for moisture to enter a brake system. Condensation generated from regular use, washing the vehicle and humidity are some ways water can seep into the system.
Of critical importance in determining a fluids ability to handle high temperature applications is its dry boiling point. Brake fluids are formulated with boiling points ranging from a minimum dry boiling point of 401oF (DOT 3), to 446oF (DOT 4), to 500oF (DOT 5.1 and DOT 5). The dry boiling point is the temperature at which a brake fluid will boil in its virgin, non-contaminated state. This is opposed to the wet boiling point, which is the temperature a brake fluid will boil after it has been fully saturated with moisture.
It is not unheard of for brake calipers to exceed 212oF (the boiling point of water). So even if a small percentage of the brake fluid boils, the gas can create a vapor lock that will decrease the ability of the fluid to transmit force, which, in turn, could significantly reduce your ability to stop quickly. A typical 3 to 4 year old car that hasn't had its brakes flushed, for example, may have a moisture content of 3%. This small amount of moisture will effectively reduce the dry boiling point of DOT 3 "rated" at 401oF, to under 300oF. Many experts believe that if the brake system absorbs more than 4% moisture - which lowers the fluid close to the boiling point of water - it is only as effective as regular water. Prolonged braking, such as driving down a mountain or braking hard for Turn 1 at Summit Point, is not the best time to discover that you have brake fluid with a high moisture content.
DOT 5, on the other hand, is not hygroscopic. So as moisture enters the system, the brake fluid does not absorb it. This means its boiling point will remain relatively stable. But this results in beads of moisture moving through the brake line, eventually collecting in the calipers. It also means that any water that does get into the brake system will tend to form pure water pockets, which could exacerbate brake corrosion. If the steel pistons develop rust that prohibits them from functioning as designed, braking performance will suffer and could lead to brake fade, failure, and/or costly repairs since the brake calipers would probably need to be replaced.
Routinely changing brake fluid in your car will remove contaminants in the system, maintain a high boiling point, and prevent water from attacking and corroding internal brake parts like ABS modulators, pistons seals and other rubber components.
Flushing the Brakes
For the reasons outlined above, routinely flushing hydraulic systems every two years or 30,000 miles, whichever comes first, is recommended by almost every automotive manufacturer. Preventative maintenance is unquestionably the best and most cost effective means of maintaining your brake system. This method of replacing brake fluid should not, however, be confused with brake bleeding. The term "bleeding the brakes" usually refers to eliminating air from the system without necessarily replacing all the brake fluid. The objective is to remove just enough fluid - often by "pumping and holding" the brake pedal - in order to force the air out of the system. Conversely, the purpose of flushing the brake system is meant to completely remove the old brake fluid and replace it with new fluid.
There are generally two accepted methods of flushing the brakes: pressure bleed or vacuum bleed. Pressure bleed systems utilize a device that attaches to the master cylinder and using compressed air, forces the old fluid out through the bleeder screws in the brake caliper.
Mildmay Autmotive however, prefers the vacuum bleed method. We begin by first extracting all the old fluid from the reservoir. Whenever possible, the reservoir is also removed from the master cylinder and thoroughly cleaned. After re-installing and filling the primary reservoir, an automatic refill/secondary reservoir is then placed on top of it and filled with fresh brake fluid. The purpose of this device is to prevent air from entering the system as the fresh fluid is drawn through and out each caliper. A strong vacuum is then developed in a fluid recovery canister (using compressed air). Starting at the brake caliper farthest away from the master cylinder, fluid is then extracted from the bleeder valve/screw on the brake caliper. Once the fluid is clean and free of bubbles, this process is then repeated for each brake caliper.
When you add fresh fluid to your existing system, never mix fluids of different DOT classifications. Doing so will contaminate the brake fluid and could lower the boiling point of the new fluid. If you do plan on using a different DOT classification brake fluid, we recommend that you completely and thoroughly flush the entire system first.