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Braking System – How It Works and What You Need To Know About Servicing It

brakes replacement Mustang

The braking system in your car, truck or SUV is much more than just a set of brake pads and calipers (or brake shoes and drums depending on the vehicle) – there are three systems that work together to bring your vehicle to a complete stop. The braking itself is performed by the mechanical friction components; however the hydraulics and power assist systems must be in tip-top shape in order for the mechanical braking to work. Far too often overlooked, the braking system is the single most important system in your vehicle, failure in the brakes is not an option. Let’s start at the beginning and walk through just how your brakes work and what you need to know about servicing them.

Brake System Power Assist

All modern vehicles use some sort of power assist in the braking system. Most vehicles use a vacuum diaphragm which utilizes engine vacuum stored in a large reservoir on the firewall to assist in applying pressure to the master cylinder when you press the brake pedal. Electric vehicles like the Nissan LEAF use an electric motor to assist with braking power. Large trucks and diesel vehicles often use a hydroboost unit which is driven off the engine.  This is similar to a vacuum booster, except it uses pressure from the power steering system instead of engine vacuum. Hydroboost can also be found in vehicles where there is no room for a vacuum booster, or there is a lack of constant engine vacuum. These systems do not fail very often, but when they do, your braking effort is significantly increased, similar to when the engine is not running. Vacuum boosters do go bad over time, as the diaphragm inside the unit can tear. Similarly an electronic brake booster can fail due to electrical issues. Hyrdoboost systems are under tremendous hydraulic fluid pressure and can leak. These brake boosters are not user-serviceable, they are strictly replacement items.

Most cars feature a vacuum booster for the power assist. When these fail, you will have heavy pedal. They draw vacuum from the engine itself.

Brake System Hydraulics

The hydraulic system is the most frustrating component of any braking system. While it is simple fundamental physics, keeping a sealed hydraulic system sealed can be a serious pain in the drums. The hydraulic system is comprised of a master cylinder (the block of iron or aluminum mounted to the power brake booster on the firewall), a series of lines that run from the master cylinder (possibly through the ABS pump depending on the vehicle) to the brake calipers (disc brakes) or wheel cylinders (drum brakes). The master cylinder is actuated by the brake pedal/power booster through a piston. The piston forces the fluid through the lines and into the calipers/wheel cylinders which expand their own pistons. Because liquid does not compress, this works very well. However, even the slightest amount of air in the lines will give the brake pedal a spongy feel, and enough air in the system can negate the system’s ability to function at all. Bleeding the brake lines is typically a two person job, with one person pumping and holding the brake pedal, while the other opens and closes the bleeder valve on each caliper or wheel cylinder. This process can take several tries to get all of the air out of the system.

The master cylinder comes in many different styles, but they are all the same internally. This is where the fluid is stored and pumped through the lines.

Another common issue with the hydraulic system is the rubber flex lines. Over time, the rubber deteriorates, eventually failing, resulting in a complete loss of braking ability. Be sure to check your lines for cracks and wear on a regular basis. The lines may also swell and lose their ability to hold their shape under braking, instead of transferring pressure they will expand like a balloon. Less frequently, the master cylinder needs servicing, as the seals wear out, allowing the master cylinder to leak externally as well as internally, reducing the braking ability. An internal master cylinder leak allows brake fluid to flow around the outside of the master cylinder piston rather than pressurize the braking system.

The flex lines connect the hard lines of the hydraulics to the brakes on the wheels. These lines see a lot of movement and road debris, so they crack and split over time.

The hydraulic system is also under constant attack from the fluid itself. DOT 3 brake fluid is glycol based, which is hygroscopic, meaning it absorbs water. Even though the system is sealed, it still gets exposed to air through the top of the master cylinder. Over time, the fluid absorbs so much water that it will rust the lines and components, leading to dark brown/orange colored fluid. When brake line corrosion occurs it is actually eating away at the copper lining, leaving dissolved copper in the brake fluid. Fluid should be flushed and replaced when it becomes dark to avoid damage to the rest of the system. Ideally it should be done anytime you replace the master cylinder. You can test your brake fluid with a simple dip strip tester.

These test strips can tell you if the fluid is good or bad. Cheap insurance for your braking system.

Brake System Mechanical Components

The most common issue in a braking system is the mechanical portion. This is where all of the actual braking happens. The mechanic action is a simple friction-type clamp for both disc and drum brakes. In a disc brake, two brake pads clamp onto the rotor between the piston(s) of the calipers. These are the most efficient types of brakes. In drum brakes, a pair of brake shoes press outward into the lining of the brake drum. Drums are most commonly found on the rear axles, but many pre-1975 cars have drums on the front wheels as well. Drum brakes are simple and efficient which has helped them stay in use for rear braking even on modern vehicles.

This is a drum brake. Most modern cars have 4-wheel discs, but many trucks still use rear drums, and most older cars do too.
A common brake drum. The fins are to help provide cooling via air flow.
Inside the drum is a lever system that operates pushes the brake shoes out to the drum lining.
Inside the drum is a lever system that operates pushes the brake shoes out to the drum lining. The wheel cylinder is located in the top middle above the axle flange. The brake shoes are located on the outside perimeter. An adjuster at the bottom controls how close the brake shoes are to the drum face.
The lining of the drum is usually press fit. It is not replaceable in most cases, but unless the surface is severely worn, the lining can be turned on a lathe to make it usable again.
The inner lining of the drum is usually press fit. It is not replaceable in most cases, but unless the surface is severely worn, the lining can be machined on a lathe to make it smooth and usable again.

There are many types of brake pad/shoe linings (the lining is the actual friction material), including semi-metallic, non-asbestos organic NAO, low-metallic NAO, and ceramic. Semi-metallic pads are the most common and are very durable, but they tend to wear the rotors and linings faster. Ceramic pads are considered the best brake pads, being relatively dust free, low wear for the rotors and very low noise. The drawback is the expense, as ceramic pads are significantly more expensive than the other types. NAO and non-asbestos NAO pads create a lot of dust and wear out faster than the semi-metallic and ceramic pads.

The other components of the mechanical braking are the rotors or drums. These wear out, but not as often as the pads. When they get worn, they can be resurfaced, until they reach the minimum safety guidelines stamped into the hub. If you let that grinding noise go one too long, your rotors and drums may not be serviceable at all, and will require replacement. Your Local NAPA AutoCare Center or NAPA Auto Parts store will have the parts you need.

Brake System Electrical Components

Just about every car built since the 1990s has been fitted with ABS braking controls. This system is electrically controlled through the hydraulic system to reduce loss of control when traction is lost under braking, such as sliding on ice. The ABS system is fairly complicated and, in most cases, service issues warrant a visit to a professional. Some vehicles may have brake pad sensors which can warn the driver when the brake pad friction lining has reached replacement thickness. There are also brake fluid level sensors and brake pressure sensors to alert drivers of an issue.

Regenerative Braking

Lastly for those with an electric vehicle there is regenerative braking. Regenerative braking systems use an electric vehicle’s own motors to slow down the vehicle. The motor is briefly turned into an electrical generator which slows down the wheels. The power that is created during regenerative braking is then put back into the main battery pack. In an ordinary combustion engine fueled vehicle the energy created from braking is dissipated as heat, but in the case of an electric vehicle it can be channeled back into electricity.

Maintaining proper braking efficiency is not difficult, you just need to pay attention to how your car behaves while braking. A soft, spongy pedal, or grinding are sure signs that your braking system requires attention. Other tell-tale signs include pulling to one side or the other under braking, a hard to push brake pedal or dark colored brake fluid. Be safe and consult your local NAPA AutoCare Center whenever you need assistance.

Check out all the brake system products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on your vehicle’s braking system and how it works, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Jefferson Bryant View All

A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.

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