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Glycol Based Brake Fluids

Glycol-based brake fluids are hygroscopic, meaning they absorb water from the atmosphere. Water contamination from any source, including mechanical or accidental additions of free water, will appreciably lower the original boiling point of the brake fluid, and increase its viscosity at low ambient temperatures. Water contamination may cause corrosion of brake cylinder bores and pistons, and may seriously affect the braking efficiency and safety of the brake actuating system.

How Does the Water Get In?

Most comes from the vent in the master cylinder cap and resultant condensation in the air space above the fluid. If cans of brake fluid and master cylinders are allowed to remain open to the atmosphere for too long, it will greatly increase the amount of water absorbed. Removing the reservoir cap to check the brake fluid level also increases the exposure of the brake fluid to moisture-laden air. Water molecules will also penetrate the rubber brake hose through microscopic pores that are too small for fluid to leak out, but large enough for gasses to pass through. Some modern brake hoses now have inner liners of material designed to prevent this water incursion. Water can also get past the seals in wheel cylinders and calipers.

Where Does the Water Go?

Once the water is inside the brake system, it is absorbed into the glycol based brake fluid and dispersed throughout the system. If you are going to have water in your brake fluid, you actually want it dispersed throughout the fluid because it minimizes the chance of corrosion caused by localized pockets of water. It also prevents a pocket of water in a caliper boiling, which would occur around 212ºF, much lower than the boiling point of the brake fluid.

Brake Fluid Boiling Points

What Can Be Done to Prevent the Water from Getting In?

So long as you stick with glycol based brake fluid, you can’t stop it, but you can slow it down. To combat the hygroscopic nature of glycol based brake fluids, brake fluid manufacturers add chemicals to the glycol base compounds. Castrol was very successful in this regard; the LMA in “Castrol GT LMA” stands for “low moisture absorption.” Lockheed Super DOT 4 has similar low moisture absorption properties. Note that the brake fluid formulation can slow the rate at which water is absorbed, but they cannot stop it. Things we can do start with how we check the fluid level in the reservoir. Many British cars have metal brake reservoirs fitted with metal caps, and to check the fluid the cap must be removed. The switch to transparent brake and clutch fluid reservoirs that occurred in the 1960’s was made in part to eliminate the need to open the reservoir, but there is no easy way to retrofit your British car with a clear plastic reservoir. When you check the fluid, do it quickly. Think about the gasket in the reservoir cap. Many modern reservoirs use a flexible rubber gasket that covers the entire opening. This reduces the amount of moisture that can get to the fluid through the vented cap. Note that these gaskets have accordion-like folds so that the pressure can be equalized on both sides, and there is generally a tiny slit in these gaskets, so they do not “seal” the reservoir. [See Appendix A, 582-505] Brake bleeding procedures frequently suggest that you put a piece of clear plastic wrap over the open reservoir secured with a rubber band. It will limit the exposure of the fluid to moisture while allowing you to keep tabs on the fluid level. Choose your method of bleeding the brakes to limit the amount of moisture laden air the fluid is exposed to. Avoid using any system that puts pressurized air directly into the brake reservoir (or a remote fluid reservoir). Using a vacuum brake bleeder (See Appendix A; 056-671 Vacuum Brake Bleeder, or 386-225 Professional Air Powered Vacuum Bleeder). The important thing to remember is that although we can slow it down, we cannot stop water from getting into the brake fluid.

How Much Water Are We Talking About?

Glycol based brake fluid typically absorbs about one percent or more moisture per year of service life. Many two-year-old vehicles have as much as two to three percent water in the brake fluid. The amount of water absorbed will continue to increase over time, and the water content can reach 7 or 8%.

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