The brake pedal operates a hydraulic system that is used for 2 reasons. First, fluid under pressure can be carried to all parts of the vehicle by small hoses or metal lines without taking up a lot of room or causing routing problems. Second, the hydraulic fluid offers a great mechanical advantage-little foot pressure is required on the pedal, but a great deal of pressure is generated at the wheels.
The brake pedal is linked to a piston in the brake master cylinder, which is filled with hydraulic brake fluid. The master cylinder consists of a cylinder, containing a small piston and a fluid reservoir.
Modern master cylinders are actually 2 separate cylinders. These systems are called a dual circuit, because the front cylinder is connected to the front brakes and the rear cylinder to the rear brakes; some vehicles are connected diagonally. The 2 cylinders are actually separated, allowing for emergency stopping power should one part of the system fail.
The entire hydraulic system from the master cylinder to the wheels is full of hydraulic brake fluid. When the brake pedal is depressed, the pistons in the master cylinder are forced to move, exerting tremendous force on the fluid in the lines. The fluid has nowhere to go and forces the caliper pistons (disc brakes) to exert pressure on the brake pads. The resulting friction between the brake pad and disc slows the vehicle and eventually stops it.
Also attached to the brake pedal is a switch which lights the brake lights as the pedal is depressed. The lights stay ON until the brake pedal is released and returns to its normal position.
In disc brake systems, the wheel cylinders are part of the caliper; there can be as many as 4 or as few as 1. All pistons use some type of rubber seal to prevent leakage around the piston and a rubber dust boot seals the outer ends of the wheel cylinders against dirt and moisture.
When the brake pedal is released, a spring pushes the master cylinder pistons back to their normal position. Check valves in the master cylinder piston allow fluid to flow toward the calipers as the piston returns. As the brake pads pull back to the released position, excess fluid returns to the master cylinder through compensating ports, which have been uncovered as the pistons move back. Any fluid that has leaked from the system will also be replaced through the compensating ports.
All dual circuit brake systems use a switch to activate a light, warning of brake failure. The switch is located in a valve mounted near the master cylinder. A piston in the valve receives pressure on each end from the front and rear brake circuits. When the pressures are balanced, the piston remains stationary but when one circuit has a leak, greater pressure during the application of the brakes will force the piston to one side or the other, closing the switch and activating the warning light.
Disc brake systems also have a metering valve to prevent the front disc brakes from engaging before the rear brakes. This ensures that the front brakes will not normally be used alone to stop the vehicle. A proportioning valve is also used to limit pressure to the rear brakes to prevent rear wheel lock-up during hard braking.
Instead of the traditional expanding brakes that press outward against a circular drum, disc brake systems utilize a cast iron disc with brake pads positioned on either side of it. Braking effect is achieved in a manner similar to the way you would squeeze a spinning disc between your fingers. The disc (rotor) is a one-piece casting with cooling fins (vented rotors) between the two braking surfaces. 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 affect braking action since contaminants are thrown off by the turning of the rotor or scraped off by the pads. Also, the equal clamping action of the two brake pads tends to ensure uniform, straight-line stops. All disc brakes are inherently self-adjusting.There are three general types of disc brake:
- A fixed caliper, 2 or 4-piston type.
- A floating caliper, single piston type.
- A sliding caliper, single piston type.
The fixed caliper design uses pistons mounted on either side of the rotor (in each side of the caliper). The caliper is mounted rigidly and does not move.
The sliding and floating designs are quite similar and often considered as one. The pad on the inside of the rotor is moved into contact with the rotor by hydraulic force. The caliper, which is not held in a fixed position, moves slightly, 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.
POWER BRAKE BOOSTERS
Power brakes operate just as standard brake systems except in the actuation of the master cylinder pistons. A vacuum diaphragm is located behind the master cylinder and assists the driver in applying the brakes, reducing both the effort and travel he must put into moving the brake pedal.
The vacuum diaphragm housing is connected to the intake manifold by a vacuum hose. A check valve at the point where the hose enters the diaphragm housing ensures that during periods of low manifold vacuum brake assist vacuum will not be lost.
Depressing the brake pedal closes off the vacuum source and allows atmospheric pressure to enter on one side of the diaphragm. This causes the master cylinder pistons to move and apply the brakes. When the brake pedal is released, vacuum is applied to both sides of the diaphragm and return springs return the diaphragm and master cylinder pistons to the released position. If the vacuum fails, the brake pedal rod will butt against the end of the master cylinder actuating rod and direct mechanical application will occur as the pedal is depressed.
The hydraulic and mechanical problems that apply to conventional brake systems also apply to power brakes.
Honda uses a dual hydraulic system, with the brakes connected diagonally. In other words, the right front and left rear brakes are on the same hydraulic line and the left front and right rear are on the other line. This has the added advantage of front disc emergency braking, should either of the hydraulic systems fail. The diagonal rear brake serves to counteract the sway from single front disc braking.