When the first automobiles left the assembly line, the brake systems were mechanically operated. Today, a hydraulic system actuates the brakes. The hydraulic system transports a hydroscopic fluid from the master cylinder to the brake calipers or wheel cylinders at each wheel. A proportioning valve is used to equally distribute the fluid's pressure to each wheel providing safe and dependable stopping power.
Automobiles use hydraulics for two reasons:
Master cylinders consist of a dual chamber fluid reservoir along with a cylinder and dual piston assembly. Dual type master cylinders are designed to separate the front and rear braking systems hydraulically in case the system becomes compromised. This redundant system has the ability to isolate the front or rear brakes in case of a system leak providing braking. Master cylinders covert mechanical motion from the pedal into hydraulic pressure within the lines. This pressure is translated back into mechanical motion at the wheels by the wheel cylinders (drum brakes) or calipers (disc brakes).
Brake fluid is carried by steel lines to a point on the vehicle's frame near each of the vehicle's wheels. A flexible hose completes the fluid transfer to the wheel cylinders and calipers. Using a flexible hose allows for suspension travel and steering the vehcile without compromising safety.
Wheel cylinders contain two pistons at either end of the cylinder. The pistons push outward in opposite directions and force the brake shoes to contact the drums and stop the vehicle.
Disc brake calipers work the same way as wheel cylinders. However, instead of pushing the shoes outward, the calipers push the brake pads inward toward the rotor. There are fixed and floating calipers. A fixed caliper is solidly mounted to a bracket. These calipers typically have pistons on both sides and exert equal force to each brake pad. A floating caliper is mounted to slider pins and have pistons (normally one) on one side only. As the brake pedal is depressed, the piston in the floating caliper expands outward to the brake pad. The opposite side of the caliper is drawn toward the rotor with equal pressure, however the single piston does all of the work.
Pistons in a caliper or wheel cylinder have a seal, made of a rubber compound impervious to brake fluid. The seal prevents fluid leakage. A rubber dust boot is added the outer end of the cylinder to prevent dust and dirt from affecting the piston's operation. Dust boots fit around the outer edge of the piston on disc brake calipers, and around the brake actuating rod on wheel cylinders.
When the brake system is released, the entire system from the master cylinder to the calipers and wheel cylinders is full of fluid. Depressing the brake pedal causes fluid trapped in front of the master cylinder piston(s) through the lines to the wheel cylinders or calipers. The pistons are forced outward, in the case of drum brakes, and inward toward the disc, in the case of disc brakes. The motion of the pistons is opposed by return springs mounted outside the cylinders in drum brakes, and by hydraulic displacement in disc brakes.
Releasing the brake pedal immediately returns the master cylinder pistons to the normal position by way of springs in the cylinder. The pistons contain check valves and there are compensating ports drilled in to the master cylinder. The ports are uncovered as the pistons reach their resting position. Piston check valves allow fluid to flow toward the master cylinder as the pistons withdraw. The return springs force the brake pads or shoes into the released position and the excess fluid flows back to the reservoir through the compensating ports.
Dual circuit master cylinders have two pistons (front and rear), in the same cylinder. The primary (rear) piston is actuated by mechanical linkage from the brake pedal through the power booster. The secondary piston (front) is actuated by fluid trapped between the two pistons. If a leak develops in front of the secondary piston, it moves forward until it bottoms against the front of the master cylinder, and the fluid trapped between the pistons will operate the rear brakes. If the rear brakes develop a leak, the primary piston will move forward until direct contact with the secondary piston takes place, and it will force the secondary piston to actuate the front brakes. In either case, the brake pedal moves farther when the brakes are applied, and less braking power is available. This is commonly known as a redundant or backup system.
Dual circuit systems have a low-pressure warning switch to warn the driver of a compromised system. The switch is located in a valve body that is mounted below the master cylinder. A hydraulic piston receives pressure from both circuits. Each circuit's pressure being applied to either end of the piston. When the pressures are in balance, the piston remains stationary. When one circuit is compromised, the greater pressure from the other circuit (during application of the brakes) pushes the piston to one side, closing the switch and activating the brake warning light.
Disc brake systems incorporate a metering valve in this valve body and, in some cases, a proportioning valve. Metering valves keep pressure from traveling to the disc brakes on the front wheels until the brake shoes on rear wheels have contacted the drums. This ensures that the front brakes will never be used alone. The proportioning valve controls pressure to the rear brakes to lessen the chance of rear wheel lock-up during very hard braking.
You can check the warning light by depressing the brake pedal while someone else opens a bleeder screw on the caliper or wheel cylinder. If the light does not go on, substitute a new lamp, make continuity checks, and, finally, replace the switch as necessary.
NOTE: When opening the bleeder screw, ensure that the reservoir is full, and do not release the brake pedal until the bleeder screw has been tightened. If the pedal is released, you will need to perform a complete brake bleeding procedure to ensure there is no air in the system.
Check the hydraulic system for leaks by applying and holding pressure to the pedal. If the pedal sinks to the floor, either slowly or quickly, the system has a leak. A spongy feel to the brake pedal, without sinking to the floor, indicates air in the system.
Check for leaks from the master cylinder to the wheel cylinders. To check the master cylinder, remove the mounting nuts at the power booster and pull the master cylinder forward. If the rear of the cylinder is wet with brake fluid, replace the component. Check the metal lines along the length and at connections, and check the brake hoses. Finally, pull the dust boots back from the wheel cylinders. If brake fluid is found inside the dust boots, replace the wheel cylinders. It is also possible to have a compromised master cylinder with no apparent leaks. With this condition, the pressure is bypassed to the reservoir and not to the wheel cylinders.
Disc brakes use a rotor with brake pads, instead of a drum with expanding shoes. Much like the hand-brake configuration on a bicycle, brake pads squeeze inward against either side of the rotor.
Disc brake rotors are typically made from a cast-iron alloy. There are two types of disc rotors; solid and vented. A solid rotor is just that; there are no cooling vanes to dissipate heat. Vented rotors utilize an air space between each rotor half. As the wheel turns, the rotor draws in air from the rear of the rotor and expels the air through the center between the rotor halves. This enables air to circulate between the braking surfaces making them less sensitive to heat buildup and more resistant to fade. Dirt and water do not drastically affect braking action since contaminants are thrown off by the centrifugal action of the rotor or scraped off the by the pads. Also, equal clamping action of the two brake pads tends to ensure uniform, straight line stops. Disc brakes are inherently self-adjusting.
There are three general types of disc brake calipers:
Fixed caliper designs use four pistons; two mounted on either side of the rotor. The caliper is mounted rigidly and does not move.
Floating and sliding designs are quite similar. In fact, these two types are often lumped together in the same category. In floating and sliding configurations, the pad on the inside of the rotor is moved into contact with the rotor with hydraulic force. The caliper, which is not held in a fixed position, moves bringing the outside pad into contact with the rotor. There are various methods of attaching floating calipers. Some pivot at the bottom or top, and some slide on mounting bolts. They both, however, accomplish the same task.
A drum brake system consists of two brake shoes mounted on a stationary backing plate. These shoes are mounted inside a circular drum that rotates with the wheels. The brake shoes are held in place by springs and pivot points. This mounting allows the shoes to ratchet (servo action) within the drum creating a smooth transition of the friction material to stop the vehicle safely. The brake shoes are actuated by wheel-cylinders that are mounted at the top of the backing plate. When applied, hydraulic pressure forces the wheel cylinder's push-rods outward. Since these links bear directly against the top of the brake shoes, the tops of the shoes are then forced against the inner side of the drum. When the brake pedal is released the wheel cylinder is relaxed, and return springs pull the shoes back away from the drum.
Modern drum brakes are designed to self-adjust during brake application when the vehicle is moving in reverse. This causes a reverse ratcheting effect of the brake shoes, rocking an adjusting lever, thereby causing rotation of the adjusting screw. There are also drum brake systems designed to self-adjust during brake application at any time. Self-adjusting brake shoes maintain an acceptable distance between the drum and shoes. It also reduces the need for maintenance adjustments and keeps both the brake function and pedal feel satisfactory.
Most modern automobiles are equipped with a vacuum assist power booster. The purpose of the booster is to multiply the braking force, and reduce pedal effort. The booster draws vacuum from the base of the intake manifold, and since vacuum is generally available when the engine is operating, the system is simple and efficient. Some vehicles do not have a sufficient source of vacuum; these vehicles use a hydro boost system that is typically run by the power steering pump, or other auxiliary pump. The booster contains a vacuum diaphragm and assists the driver in applying the brakes, reducing both the effort and travel that's required to move the brake pedal.
The power booster is connected to the intake manifold by a metal line and vacuum hose. A check valve is installed where the hose enters the diaphragm housing. At periods of low manifold vacuum, there will be no loss of brake assist.
The vacuum source is closed off when the brake pedal is depressed. Atmospheric pressure enters on one side of the diaphragm, causing the pistons in the master cylinder to move, applying the brakes. Releasing the brake pedal causes vacuum to be applied to both sides of the diaphragm, and return springs move the diaphragm and master cylinder pistons to the released position.
If manifold vacuum is lost, the system will still work without any power assist. However, a greater effort must be exerted to stop the vehicle and the brake pedal will feel rock hard.