After only a year of service, the brake fluid in the average vehicle may contain as much as two percent water. After 18 months, the level of contamination can be as high as three percent. And after several years of service, its not unusual to find brake fluid that contains as much as seven to eight percent water.
An NHTSA survey found that the brake fluid in 20% of 1,720 vehicles sampled contained 5% or more water!
As the concentration of moisture increases, it causes a sharp drop in the fluids boiling temperature. Brand new DOT 3 brake fluid must have a dry (no moisture) boiling point of at least 401 degrees F, and a wet (moisture-saturated) boiling point of no less than 284 degrees. Most new DOT 3 fluids exceed these requirements and have a dry boiling point that ranges from 460 degrees up to over 500 degrees.
Only one percent water in the fluid can lower the boiling point of a typical DOT 3 fluid to 369 degrees. Two percent water can push the boiling point down to around 320 degrees, and three percent will take it all the way down to 293 degreeswhich is getting dangerously close to the minimum DOT and OEM requirements.
Water contamination increases the danger of brake failure because vapor pockets can form if the fluid gets too hot. Vapor displaces fluid and is compressible, so when the brakes are applied the pedal may go all the way to the floor without applying the brakes!
In addition to the safety issue, water-laden brake fluid promotes corrosion and pitting in caliper pistons and bores, wheel cylinders, master cylinders, steel brake lines and ABS modulators.
Ford, however, recently changed its position and now recommends fresh fluid every 36,000 miles or three yearsand to replace the fluid each time the brake pads are changed.
Several import vehicle manufacturers also recommend fluid changes for preventative maintenance. BMW says the fluid should be changed every two years. Honda recommends a flush & fill every 25,000 to 30,000 miles. Subaru also recommends a 30,000 mile fluid change. Volkswagen recommends changing the fluid every two years, and clearly states this in their owners manuals.
As for GM, we can only look at their past recommendations with DexCool that clogged and ruined many a cooling systems years ago.