The difference in dot# means only two things.
Not quite true.
White there is a difference in boiling points it is not the only thing...
Brake fluid "wet" boiling points from an old textbook:
DOT3 140C (284F);
DOT4 155C (311F);
Super DOT4 195C (383F);
DOT5 185C (364F);
DOT 5.1 185C (365F).
The increase in DOT # is based on the order they were developed and approved for use in cars (ie US
Dept.
Of
Transportation, and the "version", if you will)
DOT "1"& 2, I believe were forms of mineral oil blends with issues in braking systems, thus no longer used.
DOT 3 & 4 are glycol based, but DOT 4 includes the addition of elastomeric borate-esters, which changes the boiling point and lubricating properties, specifically developed for use with ESP and ABS systems in mind.
DOT 5 is silicone-based and is incompatible with prior fluids, as well as ABS/ESP systems and many types of rubber brake lines. It was developed as an alternative that would not absorb moisture, thus retaining the 'dry' boiling point. (typically used in power-sports or racing applications)
DOT 5.1 was developed due to DOT 5's shortcomings. It is considered the "non-silicone" version of DOT 5, despite the official definition requiring "less than 70% silicone" . It contains some portion of glycol base, which makes it much more compatible with ABS/ESP while maintaining many of the benefits of DOT 5.
The differing blends of chemicals in each type of fluid makes it a bad idea to mix them (especially DOT 5, which supposedly has a corrosive reaction with glycol based fluids) At the very least, mixing reduces the service life, but it can also dramatically swing boiling points depending on quantities involved.
Flushing the brake system
completely of any old fluid and going
up in DOT #, usually doesn't cause problems if the system is inspected for compatibility, but the benefits are usually quite low unless the use-case of the vehicle has skewed from what was intended (ie Racing) . However, going
down can cause problems: from boiling fluid, to system damage (ABS/ESP), to metal and seal incompatibility (swelling and corrosion), etc....
The easiest method is to stick with what the manufacturer recommends: they spec'd the fluid for a reason. DOT 3 was industry standard until ~2006. So if a car deviated from the standard prior to then, it was likely for a
good reason....
EDIT: From Wikipedia - DOT Standard for brake fluids
Federal Motor Vehicle Safety Standard 116 (FMVSS 116) regulates motor vehicle brake fluids in the
United States. Like all other Federal Motor Vehicle Safety Standards, FMVSS 116 is administered by the
United States Department of Transportation's
National Highway Traffic Safety Administration.
This standard specifies requirements for fluids for use in hydraulic brake systems of motor vehicles, containers for these fluids, and labeling of the containers. The purpose is to reduce failures in hydraulic braking systems of motor vehicles which may occur becuse of the manufacture or use of improper or contaminated fluid. The standard applies to all fluid use of passenger cars, multipurpose passenger vehicles, trucks, buses, trailers and motorcycles equipped with a hydraulic brake system.
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