Brake System Basics

The typical passenger-vehicle brake system basics is relatively simple. When you step on the brake pedal, the force your leg exerts is applied to a device called a master cylinder. The master cylinder contains a piston that pressurizes a network of hydraulic brake lines that lead to each of the vehicle’s wheels. At each wheel, that brake fluid pressure operates the brakes by driving pistons that force replaceable linings against a rotating drum or disc. Friction is what slows the wheel, and in turn, the entire vehicle.

Power brakes are standard on virtually all modern passenger vehicles (but not that many classics), and they use energy supplied by the engine to help power the brakes so that the strength of your right leg doesn’t have to do all the work. To eliminate the possibility of sudden, complete brake failure, modern vehicles actually have two parallel brake systems, with each system controlling two of the vehicle’s wheels. This way, even if one system has a major failure, the other system can still stop the vehicle (albeit less effectively).

Brakes themselves have dramatically improved over the years, too. A few decades ago, drum brakes were in wide usage, and they’re still used on the rear wheels of many vehicles. This type of brake employs a drum-shaped assembly that spins with the wheel. Inside the drum, stationary “shoes” faced with replaceable friction material are forced against the drum when you push the brake pedal. Drum brakes work well, but they have a hard time shedding heat well enough to prevent fade when used really hard.

Brake fade occurs when the brake overheats dramatically; braking power is vastly reduced, and the brake components and linings can be damaged. A significant advancement came in the form of disc brakes, which today are used almost universally on front wheels (which do most of the work under braking) and on many rear wheels. Disc brake systems have a metal (or exotic material in some racing applications) disc (or rotor) that spins along with the wheel, and a stationary “caliper” that squeezes the disc with replaceable friction material when the brakes are applied. With plenty of airflow on the exposed discs, these types of brakes are much less fade-prone.

Additionally, the discs are often internally vented to allow even greater airflow. Back when brake fade was a common problem on longmountainous descents, drivers would shift the transmission into a lower gear to allow engine braking to take some of the load off of the brakes. With modern brakes, this is usually no longer required, except in situations such as towing a heavy load downhill.

Calipers

This is the component that people think of first in a brake system basics. It is the part that is behind the wheels and is the most visible when checking out the cool wheels of today. The caliper is what applies the force through the brake pads and ultimately causes the rotors to stop. Calipers can be a fixed or a floating mount and can be constructed of several different materials. The floating caliper is most commonly used by the OEM’s and is usually made from cast iron in a one or two piston configuration.

A fixed mount caliper is solidly mounted using a minimum of two pistons and used primarily in the aftermarket. Most fixed calipers are constructed from aluminum and are generally considered the most efficient design due to equal clamping force on both sides of the rotor. Calipers are available on the market anywhere from 1-piston up to some companies offering 8-piston calipers. Don’t know which way to go with all of the choices…..give us a call and let an expert walk you through your options and put together the right answers for your needs.

Brake Pads

The brake pad is what presses against the brake rotor through the caliper. Brake pads use a friction material on a metal backing plate. This friction material is the main difference between brake pads when looking at pads for a particular caliper. There are pads specific to street cars and some specific to race cars. Using the right friction material for your application is very important.

Rotors

Brake rotors can come in many different shapes, sizes and design. The most common is a vented rotor with the bearing hub cast into the rotor as one piece. Rotors are also available as a slip on rotor in a one-piece or a two-piece design. The main thing to remember about rotors and size is the bigger the rotor, the more mechanical advantage it can provide. The larger the rotor means less pressure and force by the caliper is required to stop the vehicle.

Brake Hoses

The brake hose is the flexible portion of the distribution system that attaches the hardlines to the caliper or rear axle housing. The hoses are available in either rubber or stainless steel. Either is acceptable to use in a system. The stainless steel hoses expand less under application of the brakes and are more resistant to damage from road debris. The brake hoses should be kept to as short a length as possible to avoid a spongy pedal from excessive hose length.

Master Cylinder

Outside of looks, the main differences between master cylinders are single vs. dual reservoirs. When cars were equipped with four-wheel drum brakes, the system required equal pressure at all four drums. The pressure and proportioning was done through the size of the wheel cylinders. With the introduction of front disc brakes, there became a need for a dual reservoir master cylinder. Disc brake systems required higher volume and more pressure. Therefore, the reservoirs were different sizes for disc and drum. In addition, a dual reservoir master cylinder provides for a much safer brake system since the front and rear systems are completely separate.

When using a dual reservoir master cylinder, the output pressure is equal on each port and must be regulated or adjusted through an external valve. When choosing a master cylinder, it is also important to consider the bore size of the master cylinder. This is directly related to the rest of the brake system and must be chosen based on that. The best way to understand brake pressure when choosing a master cylinder is to think of a garden hose. When putting your thumb over the end of a hose, more pressure is created. At the same time, you don’t want to run out of water! The smaller the bore, the more pressure produced. Make sure you have enough brake fluid to service the calipers being used at the correct pressure.

Brake Booster

The primary function of a booster is to provide additional assist when pressing on the brake pedal. The assist that the booster provides allows less pressure to be applied to the brake pedal but still maintain proper brake pressure needed in the system. A power booster operates using engine vacuum and requires 18” of vacuum from the engine for the booster to function properly. Power brake boosters can range anywhere in size from 7” to 11” in either a single or dual reservoir configuration. The size of the booster is directly related to the amount of assist provided by the booster. What size to use is determined by available room along with the vehicle weight. It is possible to over assist a four wheel drum system but difficult to do in a disc/drum or a disc/disc system. Therefore, go with the biggest booster that will fit in your application.

Proportioning Valve

A proportioning valve reduces the amount of line pressure to the rear wheels. A proportioning valve can be either adjustable or pre-set. An adjustable valve is plumbed into the rear system and can be adjusted to set the proper brake bias for the vehicle. A pre-set proportioning valve is generally part of what is commonly called a Combination Valve. A Combination valve is the most commonly used valve in a system. It includes the proportioning valve, a pressure differential valve and a metering valve.