Twice in my career as a master auto technician, I was driving a customer’s car when the brakes failed. You never forget heart-stopping moments like that. Thankfully, the cars were in the shop because their owners noticed the brake pedal fading or felt something “funny” while stopping.
Then, this harrowing situation happened when driving my own car. Without warning, while backing out of my driveway, the brake pedal dropped right to the floor.
Luckily, I was barely moving. I quickly yanked on the parking brake and brought my SUV to a safe stop. A brake line, buried in the frame, had rusted out.
All three times I was extremely fortunate. There was no damage, and no one was injured. So why does this happen? Simple — brake fluid goes bad. Here’s what you need to know, with help from an expert — Ron Schornstein, president and CEO of Acustrip, which markets fluid testing products.
What Is Brake Fluid and What Does It Do?
Brake fluid is a solvent-based, hydraulic fluid. It contains synthetic additives formulated to withstand high pressures and temperatures when transferring force from the brake pedal to the brake pads or shoes.
A network of lines and hoses deliver the pressured brake fluid to the disc pads, forcing them to clamp down onto the disc rotor or drum brake shoes outward against the brake drums. This force creates tremendous friction that brings your car to a halt.
What Makes Brake Fluid Go Bad?
Unfortunately, the additives that protect brake fluid — lubricants, corrosion inhibitors, acid neutralizers and viscosity stabilizers — break down and wear out. Once they lose their effectiveness, brake fluid’s performance becomes significantly lower. This causes:
Moisture absorption
Brake fluid is extremely hygroscopic, meaning it can absorb and hold moisture. Moisture in brake fluid also causes metal brake parts to rust and corrode from the inside out, and rubber parts to swell, deteriorate and weaken.
The heat friction that slows rotor and wheel rotation transfers to the brake fluid. This causes the moisture to boil, producing gas bubbles. Gas bubbles in brake fluid can compress, causing brake fade (the pedal feeling mushy) and greatly increasing stopping distances.
Incompatibility
Different types of brake fluids are not always compatible with each other.
- DOT 2 brake fluid is mineral based and should not be mixed with any other DOT rated fluid.
- DOT 3 and DOT 4 are glycol ether based. You can mix DOT 4 with DOT 3 brake fluid, but never add DOT 3 to DOT 4.
- DOT 5 brake fluid is silicon based and incompatible with any other DOT brake fluid.
- DOT 5.1 is the same as DOT 4, but has a much higher boiling point.
Check your owner’s manual or ask an expert at an auto parts store for the correct type of brake fluid for your vehicle.
Note: The “DOT” acronym stands for the U.S. Department of Transportation, which sets the standards for brake fluids, including:
- Boiling points;
- Chemical interactions with other materials used in braking systems;
- Chemical and temperature stability;
- Container labels are accurately labeled;
- Viscosity levels.
Chemical breakdown (additive depletion)
Over time, exposure to extreme temperature fluctuations and tremendous pressures also causes brake fluid additives to weaken. This can make brake fluid caustic, corroding metal braking system components and plastic and rubber brake parts to deteriorate.
Contamination
Although a sealed system, debris or other impurities can enter the brake fluid hydraulic system. A small vent in the master cylinder prevents brake system vapor lock, which could seriously impact braking performance.
The vent allows air into the reservoir as the brake fluid level drops when stepping on the brake pedal, then exits when stepping off the pedal. Air entering the master cylinder contains moisture, dust and dirt, all of which contaminate the fluid.
Electrolysis
This is a little-known reason for brake fluid failure. Caused by unlike metals touching or poor body grounds, electrolysis causes parts to decompose by passing an electric current that should be going to ground through a liquid like rain, salt air or road salts.
Clamps and other hardware securing brake lines to the frame or undercarriage, made from metals different than brake lines, corrode from exposure to these elements. This causes brake lines to decay.
Poor body or component grounds can cause many problems, from broken metal control cables, rotted heater cores, lights and other electrical devices acting weird, plus engine drivability issues. If all the brake lines and hoses on your car are not easily visible, ask your mechanic to inspect them every time you bring in your vehicle for service.
How To Tell if Brake Fluid Is Bad
Check brake fluid quality and condition using test strips or a brake fluid pen tester. Depending on the tester, it can determine if the fluid contains contaminants and pH (acidity) levels that can damage the brake system.
Schornstein says unseen contaminates in any vehicle fluids can quickly cause a premature failure of that part and others as well.
When Should Bake Fluid Be Flushed?
Every two years or 30,000 miles.
Brake fluid should be clear yellowish to light brown with a slick oily feel. If the fluid is dark brown, has black gritty particles or the fill cap gasket/seal is mushy, it’s time to flush and fill brake fluid and replace the master cylinder gasket/seal.
Check brake fluid level monthly and test the condition of the fluid every six months or 6,000 miles. Checking brake fluid level and condition are easy DIY maintenance items that can prevent costly brake repairs, as well as accidents.
About the Expert
Ron Schornstein is the president and CEO of Acustrip in Denville, New Jersey. Acustrip produces several fluid testing products and devices for the transportation industry and home usage.
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