HYDRAULIC BRAKE PRINCIPLES THE IMPORTANCE OF THE BRAKE

HYDRAULIC BRAKE PRINCIPLES

THE IMPORTANCE OF THE BRAKE SYSTEM The most critical system on a vehicle is the brake system. The safety of the vehicle passengers depends on it. BRAKE OPERATION Automotive brake systems use friction to slow down, stop, or hold the wheels of a vehicle. This is accomplished using both mechanical and hydraulic forces. The driver presses the brake pedal (mechanical force), which forces a piston against the brake fluid and creates pressure (hydraulic force).

FACTORS THAT AFFECT BRAKE OPERATION There are several factors that can affect the distance required to bring a vehicle to a stop These factors include: The construction and condition of the tires. The type of road surface. The condition of the road (wet or dry, etc. ). The condition of the brakes. KINETIC ENERGY Once the vehicle begins to move, it is kept in motion by two forces: 1. The power created by the engine. 2. The velocity (speed) and weight of the vehicle. The combination of these forces is called kinetic energy. Kinetic energy is the energy of motion. CONVERTING ENERGY Once a vehicle is in motion, it will stay in motion unless another force stops it. The brake system does this by changing the vehicle’s kinetic energy back into heat energy.

FRICTION Friction is the resistance of movement between two surfaces in contact. While there are many forms of friction, the friction between two dry surfaces provides the greatest resistance to motion. BRAKE FRICTION The friction between the stationary brake pads or shoes, and the rotating rotor or drum, enable a brake system to slow or stop a vehicle. T Changing the applied pressure can vary the friction between these parts. The harder the driver steps on the brake pedal, the greater the friction that occurs between the parts, resulting in a faster conversion of kinetic energy into heat energy. FRICTION BETWEEN SURFACES Friction between surfaces also depends on the material in contact. An example of this is a person playing basketball while wearing slippery dress shoes. The shoes would have such little friction with the floor of the basketball court that the basketball player would constantly be slipping and sliding.

TRACTION Just as the brake shoes and drums require friction to slow or stop the rotation of the wheels, the tires require friction to slow or stop the momentum of the vehicle. The ability of the tires to supply friction is called traction. No matter how well the brakes stop the rotation of the wheels if the tires do not supply traction, the vehicle will not stop. Therefore, it is actually the friction between the tires and the road that stop the vehicle. WEIGHT Braking ratio can be changed by a variety of factors. The most common factors affecting brake ratio, particularly on trucks, is vehicle loading and weight distribution. For example, if a truck is loaded heavily in the truck bed, the rear brakes will have to apply more force than if it were empty. This is because the brake ratio has shifted to the rear, requiring the rear wheels to provide more stopping power.

HYDRAULIC THEORY The theory of closed hydraulic systems was first developed in the 1600's by a French scientist named Pascal. These principles include: Liquid in a confined space will not compress, however air in a confined space will compress. When pressure is applied to liquid in a confined space, the pressure will be transferred equally in all directions. A hydraulic system can be used to increase or decrease force or motion. A hydraulic system is basically a system that uses a liquid to transmit motion or pressure from one point to another.

DRUM BRAKES The first modern brake design used on vehicles was the drum brake. Drum brakes use two brake shoes that expand outward to make contact with a drum that rotates with the wheel. The friction between the shoes and drum is used to stop the vehicle.

WHEEL CYLINDER The wheel cylinder is a component of the hydraulic system. It is mounted near the top of the backing plate and is responsible for expanding the brake shoes against the drum. BRAKE DRUM The brake drum is a cast iron or cast and steel composite drum that fits over the lug bolts or studs of the wheel. It has a large hole in the middle of the drum to center the drum on the axle flange. It rotates with the rear wheels and completely encloses the other brake components.

DISC BRAKES Disc brakes consist of a rotor that is sandwiched between two pads. The rotor is connected to and rotates with the wheel. When the driver applies the brakes, hydraulic pressure is applied to stationary brake pads on either side of the rotor. The brake pads press against the sides of the rotor, slowing or stopping its rotation.

DISC BRAKE ROTORS Disc brake rotors attach to the lug bolts on the wheel spindles. They are made of cast iron and come in two types. Internally Vented Rotor Solid Rotor

CALIPER OPERATION Brake calipers use one or more hydraulic pistons to clamp the brake pads against the brake rotor. Calipers contain seals to retain the hydraulic fluid and keep dirt out and have one or more bleed screws to remove trapped air from the hydraulic system. The brake caliper holds the brake pads in place. Brake calipers are fastened to the axle assembly using a variety of retainers. Fixed caliper disc brakes have opposing pistons that use hydraulic pressure to push the brake pads against both sides of the rotating brake rotor.

Caliper Operation

Floating Caliper The floating brake caliper is used when there is little room between the caliper and the wheel assembly.

MASTER CYLINDER The master cylinder is a foot operated hydraulic pump which generates the pressure used in the brake system. A master cylinder has four basic functions: Developing pressure, causing the caliper pistons or wheel cylinders to move towards the rotors or drums. After the pads and/or shoes produce sufficient friction, the master cylinder helps equalize the pressure required for braking. Keeping the system full of fluid as the brake linings wear. Maintaining slight pressure to keep contaminants (air and water) from entering the system.

Single Master cylinder

BRAKE FLUID Brake fluid is a special blended hydraulic fluid that transfers pressure to the wheel assemblies. Brake fluid ties all of the other brake system components together into a functioning unit. Brake fluid must have the ability to absorb moisture (Aeroscopic). Brake fluid that has moisture contamination will cause the brake fluid boiling point to decrease.

WARNING SWITCH, BRAKE FLUID LEVEL The brake master cylinder fluid level warning switch is an integral part of the brake master cylinder reservoir. It consists of a float containing a magnet and a switch mounted in the bottom of the brake master cylinder reservoir. • When the brake fluid in the brake master cylinder reservoir drops below a predetermined level, the floating magnet actuates the switch, causing the red brake warning indicator to illuminate. •

BRAKE LINES Brake lines and hoses transfer fluid pressure from the master cylinder to the front and rear brake assemblies. Brake lines are double-wall steel tubing that have double-lap flares on their ends. Rubber brake hoses are used where flexing action is required. For instance, rubber brake hose is used between the frame and the calipers of the front wheels. This allows the wheels to move up and down with suspension action.

Power Brakes Power brakes are used to reduce the force required to apply the brake pedal. This is accomplished through the use of a brake booster. VACUUM BRAKE BOOSTER Vacuum boosters use vacuum from the intake manifold (gasoline engine) or a vacuum pump (diesel engines).

HYDRAULIC BRAKE BOOSTERS A power hydraulic brake booster uses power steering pump pressure to help the driver to apply the brake pedal. Sometimes called Hydraboost or Hydro-boost, it uses fluid pressure, instead of vacuum, to help force the piston forward in the master cylinder.

PARKING BRAKE SYSTEMS Parking brakes are used to prevent a vehicle from rolling forward or backward when parked on an incline or decline. They can be either hand or foot operated, and are mechanically actuated through the use of cables. DRUM PARKING BRAKES The drum parking brake system uses a parking brake lever or pedal connected to a steel cable. These cables attach to a parking brake lever on the rear brake shoe. When the driver applies the parking brake lever or pedal, the cable transmits force to the rear brake shoes. This presses them against the drum, which prevents the drum and wheel from turning.

DISC PARKING BRAKES Rear wheel disc brake assemblies are basically the same as front disc brakes. However, some rear wheel disc brakes incorporate a parking brake mechanism with self-adjuster assembly. The parking brake mechanism is actuated mechanically through the parking brake lever or pedal inside the vehicle. This pulls on the parking brake cable, which is attached to the actuator lever on the parking brake mechanism.

DRUM-IN-ROTOR PARKING BRAKE Some vehicles use a “drum-in-rotor” This type of parking brake has conventional brake shoes mounted on the rear disc brake splash shield, which acts as the backing plate. The shoes fit inside the inner diameter of the brake rotor, which acts as the brake drum. The brake shoes are forced against the rotor’s inner diameter thus preventing rotation of the

BRAKE PEDAL ASSEMBLY The brake pedal assembly acts as a lever arm to apply force to the master cylinder piston. When the driver presses the brake pedal, the master cylinder piston presses against the brake fluid. As you have learned, brake fluid is not compressible. Therefore the force is transmitted throughout the brake system as hydraulic pressure. This pressure is sent to the individual wheel cylinders, enabling the cylinders to apply the brake system's friction components.

BRAKE PEDAL SWITCH The brake pedal position (BPP) switch is used by the PCM to disengage the transmission torque converter. On some applications as an input to the idle speed control for idle quality and to disengage Cruise Control. This switch also controls the brake lights. ADJUSTABLE BRAKE PEDALS Some vehicles have an optional adjustable brake to increase driver comfort. The brake pedal position is changed in phase with the accelerator pedal. The system is adjusted by pressing a dash-mounted switch that powers the driver motor mounted on the brake pedal assembly.

Longitudinally Split In a Longitudinally split (front to rear) brake system, one master cylinder piston operates the front wheel brake assemblies and the other operates the rear brake assemblies.

Diagonally Split Diagonally split (corner to corner) brake system has the master cylinder piston operating the brake assemblies on opposite corners of the vehicle.

Brake System Control Valves Many brake system use control valves to regulate the pressure going to each wheel assembly. The three types of valves are the Main Metering, Proportioning and the Combination Valve.

Metering Valve- is designed to equalize braking action at each wheel during light brake applications. A metering valve is used on vehicles with front disc brakes and rear drum brakes. The metering valve functions by preventing the front brakes from applying until pressure has built up in the system. This overcomes the rear drum brake return springs.

Proportioning Valve A proportioning valve is also used to equalize braking action in systems with front disc brakes and rear drum brakes. It is commonly located in the brake line to the rear drum brakes. The function of this valve is to limit pressure at the rear to prevent rear wheel lock up during heavy brake applications.

Combination Valve A combination Valve is a single unit that functions as a brake warning light switch, a metering valve, and a proportioning valve. All late model vehicles use this type of valve.