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The U.S. Department of Transportation issues
specifications for brake fluid. The three main types of brake
fluid now available are DOT3, DOT4 and DOT5. DOT3 and DOT4 are
glycol-based fluids, and DOT5 is silicon-based. The main
difference is that DOT3 and DOT4 absorb water, while DOT5
doesn't. One of the important characteristics of brake fluid is
its boiling point. Hydraulic systems rely on an incompressible
fluid to transmit force. Liquids are generally incompressible
while gases are compressible. If the brake fluid boils (becomes
a gas), it will lose most of its ability to transmit force. This
may partially or completely disable the brakes. To make matters
worse, the only time you are likely to boil your brake fluid is
during a period of prolonged braking, such a drive down a
mountain -- certainly not the best time for brake failure.
As a DOT3 or DOT4 brake fluid absorbs water, its boiling point
decreases. It can absorb water from the air, which is why you
should avoid opening your car's brake fluid reservoir. For the
same reason, you should always keep containers of brake fluid
tightly sealed.
DOT5 fluid does not absorb water. This means the boiling point
will remain relatively stable, but it also means that any water
that does get into your brake system will tend to form pure
water pockets, which could cause brake corrosion.
Two other important things about brake fluid: DOT3 and DOT4 eat
paint, so don't spill it on your car. Also, none of the
different types of brake fluid should be mixed. They can react
badly with each other and corrode your brake system. |
