By Alcan, BAT Auto Technical

All brake fluids must meet U.S. Federal Safety Standard (USFSS) #116. Under this standard are Department of Transportation (DOT) minimal specifications DOT 3, 4, and 5.1 for Polyalkylene Glycol Ether (glycol) based fluids, and DOT 5 for Silicum-based Polymer (silicone) fluids.

A popular misconception is that water ends up in brake hydraulic systems through "condensation". This is incorrect. Water must be present in the first place to condense. It actually finds it's way into the system through microscopic pores in flex hoses and master cylinder reservoir diaphragms, at a typical rate of 2-3% annually. This will eventually lead to corrosion and damage of brake hydraulic system components such as master cylinder bores, wheel cylinders, disc brake calipers, and Antilock Brake System (ABS) pressure modulators.

Glycol fluids are hygroscopic (they absorb moisture) for a reason. Any moisture contamination in the system will tendto be dispersed throughout the system, minimizing water concentration at any specific location.

Silicone fluids, being non-hygroscopic, will not disperse water. This can cause high concentrations of water at the lowest points of the system such as the brake calipers or low points in lines (water is heavier than silicone fluid and will collect at the low points). This presents 2 problems: localized higher corrosion rates; and more chanceof water collecting and boiling in the calipers under heavy braking, causing gassing and pedal fade.

Silicone fluids also contain about 3X as much dissolved air as glycol fluids and are about 2X more compressible, which may cause the characteristic spongy pedal feel associated with their use, cause aeration when forced through smallorifices, and make bleeding more difficult.

Regarding seal swelling concerns, some early siliconeformulations had an incompatibility problem with glycol-designed seals but that has been corrected in most current formulations. If glycol-designed seals were designed to swell with use, then every rebuilt caliper/wheel cylinder would leak when first installed.

Currently, Harley Davidson is the only U.S. vehicle manufacturer to specify silicone fluid, and that's for paint damage concerns. None recommend silicone fluid in ABS equipped vehicles.

DRY - boiling point - WET w/3% water
401° F
284° F
446° F
311° F
500° F
(humidified)=356° F
DOT 5.1
518° F
375° F

DOT 3 represents the MINIMUM standard for disc brake fluid boiling points, which may vary from one manufacturer to another.

Glycol and silicone fluids are not intermixable, and will cause coagulation.

The bottom line is simple. Any brake hydraulic system, whether filled with glycol or silicone fluid, is subject to water contamination. Systems should be flushed every 2-3 years. This is an issue of time and relative humidity, not accumulated mileage.

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