Many manufacturers recommend changing brake fluid every two years. As
procrastinators, we usually wait until the brakes need major work and then
assume everything was changed. The fluid may have been changed at that time,
but not always. It depends on the type of brake work done, and operating
with old brake fluid in your vehicle can decrease brake performance and
cause expensive brake parts to malfunction sooner.
There are several types of brake fluid on the market. The most common is DOT
3. Brake fluids must meet standards set by the Society of Automotive
Engineers (SAE). DOT 3 (Department of Transport) fluid must have a minimum
boiling point of 401 degrees F, be compatible with other materials in the
brake system, have anti-corrosion properties and mix with other DOT 3 fluids
without changing chemical properties.
Some European car manufacturers specify DOT 4 brake fluid. Like DOT 3, this
is a glycol-based fluid, but it has a higher boiling point. DOT 4 boils at a
minimum of 446 degrees F. This higher boiling point is necessary on vehicles
expected to generate a lot of heat in the braking system. When brake fluid
boils, vapour bubbles are created in the sealed brake systems. When you step
on the brake pedal, the brake pressure compresses the bubbles, not applying
the brakes with full force. Braking performance decreases and the pedal
feels soft or spongy.
One of the problems with glycol-based brake fluids is that they are
hygroscopic. They attract moisture out of the air into the fluid. Leave a
can of brake fluid open on the bench or leave the cap off your brake master
cylinder and the fluid becomes contaminated with water. Moisture can even be
absorbed slowly through the seals in good braking systems. Water boils at
only 212 degrees F (100C), so this significantly lowers the boiling point of
the brake fluid. DOT 3 fluid with only 3% water in it boils at only 284
degrees F. This is 115 degrees lower than clean brake fluid. DOT 4 fluid
boils at 311 degrees F, 135 degrees lower than clean DOT 4 fluid. You
probably wouldn't notice this in every day driving, but if you had to brake
hard, then the additional heat in the brake system could boil the fluid and
you find yourself with poor performing brakes.
Water in the brake fluid also causes corrosion. Parts such as wheel
cylinders and brake calipers are not too expensive to replace, but those ABS
hydraulic units can easily add a thousand or more to the repair bill.
Changing the fluid will not only improve braking performance but prolong the
life of hydraulic brake components too.
Silicone brake fluid, usually referred to as DOT 5, doesn't absorb moisture
from the air, and it won't attack painted surfaces like glycol-based fluids
if you spill it on the fender or grille, but it is not without faults. The
viscosity of silicone brake fluid is different than DOT 3 and DOT 4 fluids,
so most manufacturers don't recommend it for use in systems with ABS brakes.
The biggest disadvantage of silicone fluid has to be that it is more
difficult to seal. DOT 5 fluids will seep past seals and gaskets that would
normally contain DOT 3 and DOT 4 fluids.
One of the big advantages of DOT 5 silicone fluid is it has a higher boiling
point than DOT 4, boiling at 500 degrees F. Silicone brake fluid and
Glycol-based fluids should not be mixed. The complete system needs to be
disassembled and cleaned before filling it with silicone fluid.
What's new in brake fluids? Non-silicone based synthetic brake fluids.
Sometimes referred to as DOT 5.1, it is compatible with DOT 3 and DOT 4
brake fluids. This fluid is advertised as "synthetic" but has much the same
characteristics as DOT 3 and DOT 4 fluids. This biggest advantage is its
higher boiling point of over 500 degrees F, similar to silicone-based
fluids. As long as the container says it is compatible with DOT 3 or DOT 4
fluids, you can mix it or top up the brake system with this synthetic fluid.
Better yet, take the vehicle in for service and have the old fluid bled out
and new fluid installed in your brake system. The brakes will perform
better, and you will have delayed the onset of more costly brake repairs.
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