DOT 3 and DOT 4 brake fluid are glycol based and absorb moisture over
time. Moisture contamination causes the fluid boiling point to drop
(which could lead to fluid boil and brake fade during hard use).
Moisture contamination also contributes to internal corrosion in the
calipers, wheel cylinders, and steel brake lines. The reason for
replacing fluid periodically is to get rid of contaminated fluid and
restore fluid heat and corrosion resistance.
Moisture seeps in through microscopic pores in rubber seals and
hoses. It also enters every time someone opens the master cylinder
reservoir to check the fluid level. Most fluid reservoirs are
transparent so level can be checked without having to open the lid.
Brake fluid is so hygroscopic (attracts water) that leaving the lid
off a can of fresh brake fluid can ruin it overnight. It will absorb
so much moisture from the air that it becomes too badly contaminated
to use.
How often should the fluid be replaced? By the time a new car
is only 12 months old, its brake fluid contains about 2% water.
After 18 months, the water content is approaching 3%, which is
enough to lower the boiling temperature by 25%. After several years
of service, it is not unusual to find brake fluid containing seven
to eight percent water. For this reason, many experts recommend
replacing the fluid as a preventative maintenance service every two
years or 24,000 miles. At the very least, it should be replaced when
brakes are relined.
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