What is brake fluid?
When you push down on the brake pedal, brake pads, coated with friction material squeeze, the iron brake rotors. Because the brake pads are fixed to the suspension, and the brake rotors rotate with the tires, this forces the car to slow down. A hydraulic system is used to transmit the pressure from your foot to the brakes. A piston inside the brake master cylinder (mounted to the firewall just to the right of the engine) compresses brake fluid into a network of steel pipes and flexible hoses that lead around the car. This pressurized brake fluid in turn moves pistons inside the calipers or brake drums, which in turn force the pads against the rotors or drums. (Running cables or a mechanical linkage from the pedal to the brakes is simply too complex or ineffective. Although cables and mechanical linkages were used up until the 1930’s, car performance was more modest and speeds were lower). Your vehicle almost certainly uses DOT3- or the heavier-duty DOT4-rated fluid. Automotive brake fluid is actually closer to alcohol than mineral-oil based hydraulic fluid. Very few vehicles use a mineral-oil based fluid, generally available only at the dealership. Do not attempt to substitute one for the other.
What are the symptoms of old brake fluid that would make me need to have it flushed and changed?
A few car manufacturers recommend that the original brake fluid, added at the factory on the day your car is built, never needs to be changed. Others specify an interval-based flush, others on mileage. Just to muddy the water, Toyota says that at least one model never needs fresh brake fluid, while an otherwise identical car badged as a Lexus requires a bi-annual change. Brake fluid maintenances is one of the few instances you can deviate from the manufacturer's’ suggested change interval. Here’s why:
Brake fluid is hygroscopic; it absorbs moisture from the air. As the percentage of moisture creeps up while the fluid is in service, its boiling point drops measurably. Most of the time, that’s not a big issue. Repeated panic stops or long, downhill grades can make the brake system overheat to the point of actually boiling the fluid.
Dirt and dust can infiltrate its way into the master-cylinder reservoir. In addition, as the master cylinder is actuated, rubber seals and particles of metal from the piston and master cylinder can contaminate the fluid. Eventually, these contaminants can compromise the Anti-lock Braking System (ABS), which is costly to replace. Your first inkling that your brake fluid should have been changed some time ago won’t be a soft or low pedal, but more likely an ABS warning light or the brake-warning light next to it. Generally, the brake-warning light will come on to announce a leak, and by then it’s too late. Worse yet, boiling fluid caused by heavy brake usage combined with water-contaminated fluid could leave you with virtually no brakes, halfway down a steep hill.
As a general rule of thumb, brake fluid should be changed every two years or 30,000 miles. Your repair shop should be able to use either test strips or an electronic meter to determine the level of contamination of the brake fluid in your reservoir.
What are the consequences of old, contaminated brake fluid?
Brake fluid that has soaked up too much atmospheric water will have a severely depressed boiling point, which can lead to total brake failure under extreme braking. In addition, excess moisture promotes corrosion in the rest of the brake system, potentially causing rusted-out steel lines to leak. Brake fluid contaminated with dust and wear particles can accelerate the wear of rubber seals, causing caliper or master-cylinder leaks. ABS systems may fail if the brake fluid is contaminated with dirt.
A routine pad replacement won’t include flushing. Any brake system repair that involves opening up the brake-hydraulic system, like replacing a caliper, wheel cylinder or brake line will have the cost of a flush built in, because the system needs to be purged of air afterwards, and that’s accomplished using brake fluid to flush the air out. If you aren’t having parts replaced, a simple flush should run $80-100, including the price of a fresh can of the appropriate brake fluid. Some vehicles require the use of a factory-style scan tool to purge air from the ABS pump, so you may be asked to pay slightly more for the more complex procedure.
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