This is from a internet site. Wayne
BRAKE FLUID FLUSHING -
WHAT'S STOPPING YOU?
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 Silicon-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 tend to 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 chance of 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 small orifices, and make bleeding more difficult.
Regarding seal swelling concerns, some early silicone formulations 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.
FLUID TYPE DRY - boiling point - WET w/3% water
DOT 3 glycol 401° F 284° F
DOT 4 glycol 446° F 311° F
DOT 5 silicone 500° F (humidified)=356° F
DOT 5.1 glycol 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.
batauto.comİMay 2001
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This archive was generated by hypermail 2b29 : Sun Jan 06 2002 - 22:26:50 PST