Every brake job should include a fluid change. Still, surveys have found that half of all cars and light trucks 10 years old or older have never had their brake fluid changed — even when they’ve had the pads replaced once or twice!
The reason it’s so important to change the fluid each time is because you can’t judge the condition of the brake fluid by its appearance alone. Some fluids darken as they become contaminated with moisture, while others don’t discolor. The only way to know for sure is to test it with a brake refractometer, chemical test strips or an electronic tester.
Brake fluid is hygroscopic and absorbs moisture through tiny pores in brake hoses and seals. Over time, it can pick up quite a bit of water, which increases the fluid’s viscosity, lowers its boiling temperature and increases the risk of internal corrosion in the brake system.
Tests have shown that, even after only a year of service, brake fluid in the average vehicle can contain as much as 2% water. After 18 months, contamination levels can reach 3% and continue to climb to 8% or more as time goes on. As the amount of moisture in the fluid goes up, the boiling temperature comes down. Corrosion inhibitors in the fluid also break down, increasing the risk of internal corrosion inside the brake system.
The type of brake fluid required depends on the vehicle application. Always follow the vehicle manufacturer’s recommendations. Most domestic and Japanese cars and light trucks require DOT 3 fluid, while most European cars use DOT 4. But there are exceptions, so check the owner’s manual for the requirements. Also remember that DOT 5 silicone fluid should never be used in any vehicle equipped with ABS.
While we’re on the subject, no parts in an ABS system should have to be replaced unless something has failed.
Wheel-speed sensors can sometimes give bad readings if their magnetic tips become contaminated with metallic debris from the brakes. A simple cleaning may be all that’s needed to eliminate the problem.
Unfortunately, some wheel-speed sensors are sealed inside the wheel bearing assembly and cannot be replaced unless the entire hub assembly is replaced (at considerable cost). But, if that’s the only way to get the ABS warning light to turn off so the ABS system will function normally again, that’s what you have to do.
If you remember nothing else from reading Tomorrow’s Technician, remember this: There is very little margin for error when doing brake work. Little mistakes can have big consequences. So, inspect the entire brake system end to end, identify any problems that need to be corrected, and replace all the parts necessary to restore the brakes to like-new condition.