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What turns your foot on a pedal into a force strong enough to stop your vehicle? Part of that magic happens in your brake lines. The fluid inside delivers the hydraulic pressure that activates your brakes. When your brake fluid level drops too low, your brake lines can’t build up as much pressure -- and you, your pedal, and other parts of your braking system have to work harder to stop the vehicle. If you lose too much pressure, your brakes can fail.

Hydraulic pressure in your brake lines can drop for several reasons, involving various brake components, but the three themes are:

  • Brake fluid loss.
  • Air in the brake lines.
  • Old or contaminated brake fluid.

No matter what’s going on with your brake fluid, the first step is always a Brake Inspection.

How does air get into your brake lines?

A vehicle’s brake fluid level can drop over time due to normal brake system wear as well as brake fluid leaks. Air then enters the brake lines to fill the vacuum left by lost fluid. It’s able to get in because the brake line system isn’t perfectly airtight to begin with -- and it’s open to the air every time someone checks your brake fluid level.

What happens when you have old or contaminated brake fluid?

Brake fluid, like motor oil, has additives that help fight corrosion and fluid breakdown. Just like motor oil, brake fluid is changed periodically to maintain its level of protection and performance. Another reason brake fluid doesn’t last forever is contamination. Brake fluid absorbs water. It draws moisture from the atmosphere every time it’s exposed to the air. (All it takes is removing the cap to the master cylinder to check the fluid level.) Over time, this moisture can rust and corrode the internal metal parts of your brake system.

Why is my brake fluid low?

A vehicle’s brake fluid runs low either because it’s leaking or from normal brake operation. Here’s one reason: as your brake pads and/or shoes wear down, more space opens between the pads and/or shoes, and the brake rotors and/or drums in their resting position. It takes more brake fluid to cross that bigger space and reach the wheels, thus decreasing the brake fluid level.

What happens when you have low brake fluid?

When you have low brake fluid, the effects begin with air in your brake lines (causing a spongy brake pedal), and can escalate to a critical loss of hydraulic pressure in the brake lines (causing brake failure). If your brake fluid level is low as part of normal operation, the escalation will be gradual -- but if you let it get too low, you risk getting air into expensive parts like your anti-lock brake system. If you have a brake fluid leak, there is no way to know how quickly your brake performance will deteriorate. As soon as you notice any difference in your brake response (or signs of a brake fluid leak), have your brakes inspected at your earliest convenience.

What color is brake fluid?

Fresh brake fluid ranges from colorless to light brown or amber, and older brake fluid is often a darker brown. Old brake fluid can carry debris that, over time, may erode the seals on your master cylinder and brake calipers. But color alone does not tell you when it’s time to replace your brake fluid.

Instead of relying on brake fluid color to determine when to change fluid, the Automotive Maintenance and Repair Association’s (AMRA) Motorist Assurance Program (MAP) recommends brake fluid testing by a qualified technician. Your local Midas shop can test your brake fluid for the depletion of corrosion inhibitors and for incorrect fluid added, among other things.

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  • Midas Closer Look Vehicle Check™ includes visual checks of brakes, battery, air filter, fluids, belts, and hoses.

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