Most modern cars have brakes on all four wheels, operated by a hydraulic system. The brakes can be disc or drum type.
The front brakes play a greater role in stopping the car than the rear brakes, because the brakes throw the weight of the car forward onto the front wheels.
Therefore, many cars are equipped with disc brakes at the front, which are usually more efficient, and drum brakes at the rear.
Full disc brakes are used on some expensive or high-performance cars, while full drum systems are used on older or smaller cars.
A hydraulic brake circuit has a fluid-filled master cylinder and slave cylinder connected by piping.
Master and Slave Cylinders
The master cylinder transmits hydraulic pressure to the slave cylinder when the pedal is depressed.
When you depress the brake pedal, it depresses the piston in the master cylinder, forcing the fluid to flow down the pipe.
Fluid flows to each wheel's slave cylinder and fills them, forcing the piston to extend the brake.
Fluid pressure is evenly distributed around the system.
The combined surface "push" area of all the slave pistons is much larger than the area of the pistons in the master cylinder.
Therefore, the master piston must move a few inches to cause the slave piston to move a fraction of an inch to apply the brake.
This arrangement allows the brake to exert a large force, just as a long-handled lever can easily lift a heavy object a short distance.
Most modern cars are equipped with dual hydraulic circuits, with the two master cylinders connected in series in case of failure.
Sometimes one circuit is used for the front brakes and one for the rear brakes; or each circuit uses both the front and one of the rear brakes; or one circuit works all four brakes while the other circuit works only the front brakes.
Under heavy braking, the rear wheels may shed too much weight, causing the rear wheels to lock up and possibly lead to a dangerous skid.
For this reason, the rear brakes are intentionally made less powerful than the front brakes.
Most cars now also have a load-sensitive pressure limiting valve. When hard braking raises the hydraulic pressure to a level that could cause the rear brakes to lock, it closes and prevents further fluid flow to them.
Advanced cars may even have sophisticated anti-lock systems that sense in a variety of ways how the car is slowing down and whether any wheels are locking up.
Such a system applies and releases the brakes in rapid succession to stop them from locking.
Many cars also have power assist to reduce the effort required to apply the brakes.
Usually the source of power is the pressure difference between the partial vacuum in the intake manifold and the outside air.
The servo unit that provides the assist is piped to the intake manifold.
A direct acting servo is installed between the brake pedal and the master cylinder. If the servo fails or the engine does not run, the pedal can act directly on the master cylinder.
A direct acting servo is installed between the brake pedal and the master cylinder. The brake pedal pushes a rod, which in turn pushes the master cylinder piston.
But the brake pedal also acts on a set of air valves and has a large rubber diaphragm connected to the master cylinder piston.
When the brake is off, both sides of the diaphragm are exposed to the vacuum from the manifold.
Depressing the brake pedal closes the valve connecting the rear side of the diaphragm to the manifold and opens a valve that allows air to enter from the outside.
The higher pressure of the outside air forces the diaphragm forward to push the master cylinder piston, which assists the braking force.
If the pedal is then depressed and not depressed again, the air valve no longer allows air from the outside, so the pressure on the brake remains constant.
When the pedal is released, the space behind the diaphragm reopens to the manifold, so the pressure drops and the diaphragm falls back.
If the vacuum fails due to an engine stop, for example, the brake still works because of the normal mechanical connection between the pedal and the master cylinder. But a greater force must be applied to the brake pedal to apply them.
Brakes - Both sides of the diaphragm are under vacuum.
Applying the brakes allows air to get behind the diaphragm, forcing it against the cylinder.
Some cars have an indirect acting servo installed in the hydraulic line between the master cylinder and the brake. This device can be mounted anywhere in the engine compartment without having to be directly in front of the pedal.
It also relies on manifold vacuum to provide boost. Depressing the brake pedal causes hydraulic pressure to be generated in the master cylinder, which opens the valve and triggers the vacuum servo.
Disc Brakes
A basic type of disc brake with a pair of pistons. There may be more than one pair, or a single piston operating two pads through different types of calipers (swing calipers or slide calipers), such as a scissor mechanism.
Disc brakes have a disc that rotates with the wheel. The brake disc is ridden across by a caliper, which has small hydraulic pistons that work by the pressure of the master cylinder.
The piston is pressed against the friction pads, which are clamped to the disc from both sides to slow or stop it. The pads can be shaped to cover a wide area of the disc.
There may be more than one pair of pistons, especially in a dual circuit brake.
The pistons move only a small distance to apply the brakes, and the pads barely leave the discs when the brakes are released. They have no return spring.
When the brakes are applied, fluid pressure forces the pads against the brake discs. With the brakes off, both pads barely leave the discs.
The rubber seal around the piston is designed to allow the piston to gradually slide forward as the pads wear, so that the tiny gap remains constant and the brakes do not need to be adjusted.
Many later cars had wear sensor leads embedded in the pads. When the brake pads are nearly worn, the leads are exposed and shorted by the metal disc, which illuminates a warning light on the instrument panel.
Drum Brake
A drum brake with front and trailing shoes that has only one hydraulic cylinder; a brake with two guide shoes has one cylinder for each guide shoe and is mounted on the front wheel of a full drum system.
The drum brake has a hollow drum that rotates with the wheels. Its open back is covered by a fixed backing plate with two curved shoes with friction linings.
A hydraulic moving piston in the brake wheel cylinder pushes the shoes outward, thus pressing the linings against the inside of the drum to slow or stop braking.
When the brakes are applied, the shoes are pushed against the drum by the pistons.
Each brake shoe has a pivot at one end and a piston at the other. The guide shoe has a piston located at the leading edge relative to the direction of drum rotation.
The rotation of the drum tends to pull the guide shoe tightly toward the drum on contact, thus improving braking effectiveness.
Some drums have dual guide shoes, each with its own hydraulic cylinder; others have a guide shoe and a drag shoe - with the pivot at the front.
This design allows the two boots to be forced apart from each other by a single cylinder with a piston at each end.
It is simpler but less functional than the two boot shoe system, and is usually limited to the rear brake.
In either type, when the brake is released, the reset spring pulls the shoe back a short distance.
The adjuster makes the shoe travel as short as possible. Older systems had manual adjusters that needed to be turned from time to time as the friction linings wore out. Later brakes were automatically adjusted by ratcheting.
If drum brakes are used repeatedly over a short period of time, they may fade - they heat up and lose efficiency until they cool again. The disc construction is more open and less likely to fade.
Hand Brake Mechanism
Hand brakes act on the brake shoes through a mechanical system separate from the hydraulic cylinder that consists of levers and arms in the drum; they are operated by a cable from the hand brake lever inside the car.
In addition to the hydraulic brake system, all cars have a mechanical hand brake that acts on both wheels - usually the rear.
The hand brake will provide limited braking if the hydraulic system fails completely, but its primary use is as a parking brake.
The hand brake lever pulls a cable or pair of cables connected to the brake through a set of smaller levers, pulleys and guides, the details of which vary from car to car.
A ratchet on the handbrake lever holds the brake after use. A button disengages the ratchet and releases the lever.
On drum brakes, the hand brake system presses the brake linings against the drum.
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