The Ins and Outs of Hydraulic Brake Fluid

June 10th, 2022 by

Brake fluid is shown being poured into a car's brake fluid reservoir.

While most drivers don’t spend too much time thinking about their brake fluid, it plays a big part in the overall driving experience. As much as we like to obsess about speed and performance, the ability to come to a stop is, some might argue, just as important, if not more. That’s why it’s essential that you keep up with your car brake service.

But just how do modern braking systems work? The average vehicle horsepower has increased by more than 110 percent since 1980, but what exactly is keeping all those ponies in check? Hydraulic brake fluid plays a huge role in the braking systems of many of today’s vehicles, allowing drivers to exert enough force to activate the brakes without them having to put the pedal through the floor. But just what is this mysterious liquid? Does it need to be replaced, and if so, how often? We’ll answer those questions and more as we take a deep dive into everything you need to know about brake fluid.

What Is It?

To understand what brake fluid is, it helps to understand how the brakes on today’s vehicles actually work. The simple act of pushing your foot down on the brake pedal doesn’t actually provide enough force to activate the brakes and bring a vehicle to a stop, but that’s where brake fluid comes in. Modern brake systems use hydraulics to increase the force created when you step on the pedal, allowing drivers to create the necessary energy by forcing fluid through narrow brake lines. This is a lot like a vehicle’s power steering system, which also uses a hydraulic fluid (or electric motor) to amplify the force you’re applying to the steering wheel.

It all starts with the brake pedal itself. When you press your foot down, the mechanical force is transferred from the pedal to a pushrod, which is amplified by the brake booster, which in turn transfers the force to the braking system’s master cylinder. This cylinder is full of brake fluid, and as the pushrod extends into the cylinder, the mechanical force that originated in your foot is converted into hydraulic pressure in the fluid itself. The harder you press down on the pedal, the more pressurized the brake fluid becomes. This pressure is then fed through the brake lines to each wheel of the vehicle, where the hydraulic pressure compresses the brake calipers, creating friction and slowing down the wheels.

Brake fluid is an essential part of the system as it’s essentially the medium through which the hydraulic pressure travels on its way from the master cylinder to the individual brakes themselves. Some large vehicles use air brakes that operate on largely the same principle but use compressed air instead of brake fluid. These air brakes have their advantages when it comes to 18-wheelers and other big rigs but are a little overkill when it comes to vehicles under 10,000 pounds.

When it comes to composition, most brake fluids are glycol-ether-based or, occasionally, silicone-based. Different vehicles require different types of brake fluid, and in the U.S., these break down into distinct designations: DOT 3, DOT 4, DOT 5, and DOT 5.1. These brake fluids are approved by the Department of Transportation (hence the “DOT”) for use in the U.S. but in order to determine which one your vehicle needs, you’ll need to consult the owner’s manual. These fluids are generally not interchangeable (aside from DOT 4, which can be used in the place of DOT 3), especially DOT 5, which uses a hydrophobic, silicone-based formula.

Brake fluid doesn’t just act as a medium to transfer energy throughout the hydraulic braking system; it also includes a number of additives that go a long way in protecting the system and reducing wear and tear. These include anti-corrosion inhibitors, anti-wear additives, anti-rust additives, acid-neutralizing or pH-balancing additives, anti-foaming additives, and viscosity stabilizers. When these compounds start to break down, it can have a dramatic effect on the entire braking system, which can quickly start to break down as rust, wear, and corrosion take their toll.

A closeup of a car's wheel showing the brakes is shown.

How Often Does It Need To Be Changed?

While there’s no one-size-fits-all answer to this question, as it depends on mileage and how much strain the brakes typically experience, it’s generally a good idea to get your brake fluid changed every four to five years. This varies by manufacturer and model, with brands like Chevy calling for a brake fluid flush every 45,000 miles, while Volkswagen and Mercedes-Benz play it a little safer at two years or 20,000 miles. Some manufacturers don’t include a specific replacement schedule at all and instead just recommend checking brake fluid levels periodically to ensure you’re topped off. While that sounds great in theory, we can’t in good conscience recommend driving with the same brake fluid for more than five years. In such cases, consult your mechanic, who can run a test to determine the health of your brake fluid and recommend a flush schedule that takes your driving habits and mileage into account.

As far as checking brake fluid, we like to recommend giving everything under the hood the once-over during every oil change. Checking your brake fluid, unlike changing it, is an easy, safe task that takes just seconds to complete. Simply open your hood and locate the brake fluid reservoir, usually located in the rear section of the engine compartment, relatively close to the brake pedal itself. These brake fluid reservoirs are often translucent and feature a raised line indicating what constitutes a “full” tank. Older vehicles––we’re talking 1980s and earlier––might have a metal tank instead of a translucent plastic one, requiring you to open it up and locate the “full” line on the interior, but these are not encountered too often these days. Do a quick visual inspection of the fluid itself. Healthy brake fluid should appear light brown or clear in color, so if it’s discolored or there is any particulate matter floating in it, you’re overdue for a flush.

If the tank isn’t filled to the required level, the tank can be topped off, but proceed carefully. In addition to some of the potentially harmful components of the additive packages, the fluid itself is a mixture of toxic alcohols such as ethylene glycols and glycol ethers. Brake fluid is also flammable, so take precautions when handling it and ensure the vehicle is turned off. Exposing the brake fluid reservoir to the air can create a number of problems, so make sure you actually need to top off the fluid before removing the cap, as it allows potentially damaging moisture to enter the hydraulic braking system.

Signs It Needs To Be Changed

In theory, the braking system is closed and shouldn’t lose pressure unless there’s a leak, but brake fluid does begin to break down over time owing to a number of factors. These include overuse of brakes while hauling heavy loads, frequent stop-and-go driving, overheating, or the use of cheap, low-quality brake fluid. As brake fluid either leaks or ages as the additive packages start to break down, the system might start throwing up some red flags, starting with the pedal itself.

If your brake pedal isn’t as responsive as it once was and has a distinctly more soft, squishy feel than it did initially, a lack of brake fluid is usually to blame. While it can be tough to gauge what your brakes feel like now versus how they felt when you first purchased the vehicle, one good rule of thumb is that you should never have to press the pedal all the way to the floor to bring the vehicle to a stop.

Drivers will also often notice a harsh smell of burning rubber or metal as the brakes begin to overheat, which is cause for alarm. If you notice a sharp burning smell emanating from your brakes, pull over as soon as possible and let the brakes cool before continuing on. Failure to do so can result in damaged wheels or braking systems, which can warp or even break due to excessively high temperatures.

Sound is another good indication of brake health. Old, worn-out brake fluid doesn’t allow brakes to operate as well as they should, often resulting in a loud squealing noise of metal-on-metal. There are a number of factors that could create such a racket, including worn-out brake pads and rotors, but if those check out, then the brake fluid itself is a likely culprit. Lastly, there’s the good old’ dummy light. All new cars sold since 2004 have government-mandated anti-lock brake systems (ABS), which need a certain amount of brake fluid to operate correctly. If the system detects a problem, such as a lack of brake fluid, expect the bright ABS light to activate on your dashboard.

A mechanic is shown performing car brake service.

Stay on Top of Your Brake Service

As we mentioned above, brake fluid is a nasty substance. Flammable and toxic, it’s not something you want to start messing around with, especially if you don’t have the proper tools and training. Checking brake fluid is one thing, but when it comes time for the inevitable flush and refill, we recommend leaving it to the experts. Not only do they have the expertise necessary to completely drain the system, they can also perform a test to determine just how much the brake fluid has broken down, which can be a valuable clue when it comes to diagnosing other brake-related issues.

At Thomas Nissan, we offer a full slate of brake-related services, from testing brake fluid levels and performing full brake fluid flushes to inspecting and replacing worn-out brake pads and rotors. Our in-house service center is staffed by some of the most experienced and trustworthy technicians in the area, and we’ll work tirelessly to ensure your vehicle stays in tip-top shape for years to come. Call or stop by today to schedule service, browse the inventory, or just pick our brains about some of the exciting new models on offer.

Posted in Car Brake Service