Your Vehicle's Hydraulic Brake System

Team BlueStar Inspections

Whether you are approaching a red light, a deer jumps out in front of you, or you are driving in stop and go rush hour traffic, you depend on your brakes to safely slow you down or bring you to a quick stop. The average driver applies their brakes over 200 times every day. Brakes are the single most important safety system your vehicle has.

There are hundreds of individual parts in your vehicle's hydraulic braking system. The major brake system components include the brake pedal, brake booster, brake master cylinder, brake lines and hoses, brake calipers and pistons, disc brake pads or brake shoes, disc brake rotors or brake drums, brake fluid, anti-lock brake system (ABS) control module, wheel speed sensors, and there are many more individual parts within the above component groups.

The brake pedal is designed in such a way that it can multiply the force from your leg several times before any force is even transmitted to the brake fluid. The brake pedal provides instant control over the brakes being applied and released.As you press the brake pedal, the force generated by your leg is amplified several times by mechanical leverage and then amplified further by the action of the brake booster. The mechanical force of pressing the pedal is converted into hydraulic force by the brake master cylinder which forces hydraulic brake fluid around the entire braking system within a network of brake lines and hoses. This force is transmitted to all four tires and creates friction between brake pads and disc brake rotors. This is what stops your vehicle.The brake booster, also known as the brake servo, increases the force applied by the brake pedal via either vacuum from the engine (or a vacuum pump on diesels), or via a hydraulic pump. Without the brake booster, the brakes feel very hard and require much more effort to slow the car. The booster only works when the engine is running.

The master cylinder then converts the action of you pressing on the brake pedal into hydraulic pressure. As you press the pedal it moves pistons within the cylinder which in turn applies pressure to the brake fluid forcing it around the system. The master cylinder has a brake fluid reservoir attached to the top of it to ensure there is always an adequate supply of fluid in the system whether the brakes are applied or released.

Brake lines and hoses consist of a series of thin metal pipes which connect the various components together to transfer the brake fluid around the system. Most of the pipes are made of metal, however, the area where the pipes meet the brake calipers needs to consist of flexible rubber hoses to allow the wheels to turn.

Brake calipers come in many shapes and sizes and employ one or more hydraulically actuated pistons which force the brake pads into contact with the disc rotor when the brake pedal is pushed. The more pistons a caliper has, the more evenly distributed the braking force is across the pad, and the larger the pad surface can be. The larger the pad, the greater the friction acting on the disc rotor, which equals better stopping power.

Brake pads are fitted in pairs to each disc brake rotor. They are made of a hard-wearing compound that provides excellent heat resistant properties and the ability to provide a high level of friction against the brake disc. Brake pads gradually wear away every time you apply your brakes. In addition to normal wear, brake pads can become loose, cracked, broken, and unevenly worn.

Disc brake rotors are metal discs, which also wear, but at a much slower rate. These metal discs are located between the wheel and hub and provide the friction surface for the pads to act against. Brake rotors can either be solid (one piece) or vented (effectively two discs joined by a series of veins) which aids in cooling. Vented discs are commonly used on the front of cars where the braking forces are higher and are subject to greater temperatures. Disc brake rotors can become grooved, rusted, pitted, glazed, cracked, and warped from the continual heat and pressure from the braking.

Brake drums and shoes are not common on modern vehicles, but they are still fitted to the rear of some vehicles. The brake shoes are housed within the drum, and pressing the brake pedal actuates a wheel cylinder which forces the shoes outward onto the inner edge of the drum and slows down the vehicle.

The basic idea behind any hydraulic system is force applied at one point is transmitted to another point using an incompressible fluid. Brake fluid is the incompressible fluid used in the brake system. Brake fluid efficiently operates under high pressure and high temperature and is the hydraulic fluid responsible for actuating the brake calipers or wheel cylinders at all four wheels.

The anti-lock brake system (ABS) detects when a wheel locks up when braking. The system consists of a control module, wheel speed sensors, valves, and a pump. The ABS control module monitors each wheel speed sensor and detects when one or more wheels are no longer rotating. The module uses valves and a pump to pulse the brakes on-and-off incredibly quickly (up to 15 times per second). You feel this sensation through the pedal as a hard vibration or pulsing sensation. If a fault develops in any part of the ABS, a warning light will usually be illuminated on the vehicle instrument panel, and the ABS will be disabled until the fault is rectified.

To ensure safety and reliability, brake inspections should be part of your vehicle's ongoing regular maintenance. This should include an actual visual inspection measuring pad and shoe thickness, checking for even wear of the pads and shoes, inspecting rotors and drums for warping and damage, and checking hardware to make sure it is working properly and adjusted properly. Make sure the hydraulic components including the wheel cylinders, calipers, brake lines, brake hoses, and brake master cylinder are not leaking. Inspect the level and condition of the brake fluid. Inspect for proper routing and placement of the brake lines, brake hoses, and antilock brake sensors.

Be aware of the following symptoms while braking: pulling left or right, your brake pedal is soft or low, the brake pedal slowly bleeds down when pressing it, a shuddering type of vibration or pulsation, a squealing or screeching noise, a brake warning light on the dashboard, longer than normal stopping times, a grinding type noise, an ABS light on the dash, a hissing noise when braking, or a loss of grip when braking. If you notice any of these symptoms, have your brakes inspected by an ASE certified technician. Remember, your brakes are the single most important safety system your vehicle has. Maintaining your braking system is essential for your safety, the safety of your passengers, and the safety of others.