Q: Several years ago at one of those quick oil change places, I was told that my brake fluid was old and the system would have to be flushed. That was the first that I ever heard of this in about 50 years of driving. A couple of days ago, my Hyundai dealer told me that my fluid was looking bad and recommended that it be replaced at 30,000 miles. When I questioned him, I was told that the aluminum calipers reacted with the brake fluid causing the fluid to break down. It doesn't make sense, to me, that incompatible products (aluminum and brake fluid) would be combined without one of them being made compatible with the other. What's your take on replacing the brake fluid?

—Bob Schwamm Ocala, Fla.

A: The aluminum caliper story doesn't make much sense to me either, but it's still a good idea to renew brake fluid perhaps every three or four years. Brake fluid is hygroscopic, meaning that it attracts moisture. Over time, airborne moisture sneaks into your brake fluid, which can lead to brake component corrosion and fluid fade. An antilock braking system-equipped vehicle's hydraulic control unit is very expensive and worth protecting. It's odd some carmakers don't specify brake fluid replacement in their maintenance schedule.

The most important reason for periodic fluid changes is to insure the fluid doesn't boil during heavy brake operation. Your brake system uses hydraulic (liquid) force to apply the brakes and the brake fluid absorbs some of the heat produced by the brakes. Liquid is not compressible, but a boiling vapor is. Should the water mixed in with your fluid turn to vapor, the pedal will turn to mush and the car may fail to stop.

New brake fluid has a boiling point of 401 degrees (DOT-3 fluid) or 446 degrees (DOT-4 fluid). When either fluid type becomes diluted to 3 percent or 4 percent moisture (water) content, the boiling point is reduced by about 150 degrees, increasing the chance of fluid fade. For an around-town grocery getter this isn't a huge issue, but with higher speed driving and/or mountainous or towing conditions, it could become a really big problem.

Flushing/renewing brake fluid typically involves releasing/removing fluid at each wheel's brake caliper or wheel cylinder bleeder valve, utilizing a suction generating tool. The bleeder screws resemble small, hollow bolts and are sometimes stubborn to open. ABS vehicles often require unique bleeding procedures. It's best to leave this job to a pro unless you can obtain and follow the appropriate service information, and have access to a vacuum bleeder.

Those living in a high humidity area and planning to keep their car for many years might go for the three-year renewal schedule. I confess to going perhaps four or five years on my own vehicles in a fairly dry climate. I package the brake fluid service with coolant renewal, new belts, hoses, fuel filter and all driveline fluids. This is also a good time to scrutinize battery condition, suspension and steering components, and other parts that aren't listed in the maintenance schedule.



Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood(at)earthlink.net; he cannot make personal replies.


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