Design and Function

Design and function

Wheel brakes, general









The vehicle has disc brakes on all four wheels to ensure optimum braking performance. The foot brake is combined with the parking brake on the rear axle.
The brake discs are ventilated on the front axle and are available in different diameters depending on engine type. There are three different types of rear brake discs.
- ventilated disc for electric parking brake
- solid disc for electric parking brake
- solid disc for foot operated mechanical parking brake.
The brake caliper (1) consists of two parts, housing and holder. The caliper grips across the disc brake and slides in the holder on two sliding pins (4). The sliding pins are greased and protected with a rubber bushing (3). The braking force from the brake pads (5) is absorbed via the holder (2), which transfers the force to the spindle.
The sliding pins are made of steel. They are screwed into the holder, which is secured to the stub axle.
The brake caliper contains a piston (8) together with a sealing ring (10) and dust boot (11). The piston cylinder is made of steel.
The piston presses directly on the internal brake pad. Based on the action-reaction principle, the caliper (1) presses backwards. Through this movement, the outer brake pad presses against the brake disc.

General foot brake, front and rear
When braking, the driver presses the brake pedal and an increased hydraulic fluid pressure is generated in the brake system. The increased pressure presses the piston and the inner brake pad against the brake disc at the same time as the brake caliper and the outer brake pad are pressed against the brake disc.
The dust boot (11) prevents dirt getting in between the cylinder and piston
The splash guard prevents water and salt from soiling the brake disc. Any soiling results in reduced friction and hence impaired braking power.
Certain vehicles have an electric parking brake on the rear wheel brakes. The electric motor is on the caliper and affects the piston mechanically, which then presses the pads together.

Sealing ring foot brake, front and rear

Braking




The sealing ring (10) is tensioned by means of the piston being pressed against the brake disc.

Resting position




The tension in the sealing ring (10), which was previously generated by applying the brakes, causes the piston to be pushed back to the resting position. When the brake pads wear, the rubber seal cannot follow, instead the piston slides through the sealing ring until the brake pads come into contact with the brake disc. This produces a self-adjustment between brake disc and brake pad.

Hydraulic braking system









The hydraulic brake system is a two-circuit diagonal brake system. The hydraulic brake system is divided into a primary and secondary circuits.
The primary circuit (1) includes:
- the rear chamber for the master cylinder
- the cylinder in the front right-hand brake caliper
- the cylinder in the rear left-hand brake caliper.
The secondary circuit (2) includes:
- the front chamber for the master cylinder
- the cylinder in the front left-hand brake caliper
- the cylinder in the rear right-hand brake caliper.
Two cables run from the master cylinder (3) to the brake control module (BCM) (4), where the brake pipes then branch to the wheel brakes.

Brake fluid reservoir with level switch




1. Chambers
2. Plastic floats
The brake fluid reservoir comprises two chambers (1), one on each brake circuit, which are filled with brake fluid. In the event of a rupture in one of the brake circuits, only some of the brake fluid in the system can escape. The remaining brake fluid acts on the operating brake circuit.
The reservoir contains a level function for detecting brake fluid loss in the brake system. The plastic float (2) floats on the brake fluid. This closes the connector on the switch when the brake fluid level drops below the MIN level on the reservoir. The stop (brake) warning lamp lights on the driver information module (DIM). The float and the switch are integrated with the container and cannot be replaced.

Power brake booster

Overview of the passive power brake booster









The vehicle is equipped with a double power brake booster. The power brake booster is located between the brake pedal and the master cylinder and is actuated directly by the brake pedal. It ensures that less pedal force is required when braking with the aid of vacuum from the vacuum pump and via the intake manifold for the engine. The check valve located by the vacuum pump prevents air from flowing back to the power brake booster.

Resting position




1. Max. negative pressure
2. Atmospheric pressure
In the resting position, the power brake booster components are in the positions illustrated. The push rod spring keeps the push rod and the jointed valve piston mounted on this pressed to the right. The movement is restricted by the stop washer. The valve piston holds the valve lifted out of the seat in the guide housing. The air duct is closed and the vacuum duct exposed. The same negative pressure is therefore present on both sides of the membrane. The membrane and the guide housing are pressed to the right-hand limit position by the membrane spring.
When the brake pedal is pressed down, the rear push rod and the valve piston are moved to the left (forwards). The valve spring causes the valve plate to follow until it reaches the seat in the guide housing. The connection between the front and reverse of the membrane is closed. As the piston continues to move forward, its movement is transferred via the reaction disc and the front push rod to the brake fluid in the master cylinder.

Partial and full braking




1. Max. negative pressure
2. Atmospheric pressure
3. Negative pressure
When the driver presses the brake pedal, the valve piston moves to the left as illustrated. The seat for the valve piston leaves the valve plate and the connection opens between the reverse of the membrane and the center of the valve assembly (which is at atmospheric pressure). Air at atmospheric pressure flows in behind the membrane. There is negative pressure on the front of the membrane. A difference in pressure is built up. The difference in pressure generates a force on the membrane, causing the membrane to move to the left as illustrated.
The membrane is installed on the guide housing, which therefore follows the movement of the valve piston. The guide housing force is transferred to the front push rod through the outer part of the reaction disc. Along with the force from the brake pedal, which presses on the inner part of the reaction disc, the front push rod presses the brake fluid out of the master cylinder to the wheel brakes. The vehicle is braked.
During normal braking, when the driver presses the brake pedal to a certain level and holds the pedal in this position, the following occurs:
The valve piston stops in the partial braking position and the valve seat on the moved guide housing can now close the connection between the reverse of the membrane and atmospheric pressure. No additional pressure is built up on the membrane. The force does not increase, but is now equal to the hydraulic counter pressure in the master cylinder.
If the force on the pedal is increased, the valve piston force on the center of the reaction disc increases. Some displacement of the piston will take place. The piston seat leaves the valve. More air can flow in and increased braking is achieved until the new balance position is achieved.
During full braking, when the driver presses the brake pedal all the way down, the following occurs: Air from atmospheric pressure flows behind the membrane until the maximum difference in pressure has built up. The membrane moves as far as it will go and the power brake booster reinforces the braking force applied by the driver to the maximum.
If the pedal force is reduced, the center of the reaction disc is pressed from the seat in the guide housing. The spaces on either side of the membrane are then linked to each other. The pressure is equalized, the guide housing is pushed back by the spring force and the counter force on the front push rod. Braking reduces. The valve piston can revert to the position illustrated. The new balance position has been achieved. If the pedal is completely released, all the power brake booster components return to the resting position and the brakes are released.
In the event of a fault in the vacuum supply, braking can still take place by means of the power brake booster operating as an extended push rod. As no servo assistance is obtained in this case, greater pedal force is required.

Master cylinder









The master cylinder is designed for a diagonally split brake system.
One front wheel and one rear wheel are connected together in a circuit. The two chambers in the master cylinder supply one circuit each in the brake system.
The master cylinder is of the tandem type. It contains two pistons, which are connected in series. For safety reasons, each circuit has a specific piston for pressing the brake fluid out of the master cylinder and into the brake pipes.
There is a valve (3) in the bottom of both pistons. These ensure that the remaining pressure in the chambers is released. They also ensure that new brake fluid is automatically filled into the system in the event of wear to brake pads and disc brakes.
The master cylinder's primary piston (1) is affected by the power brake booster's push rod when the brake pedal is applied. The valves are closed and pressure can build up in the brake fluid. The pressure in the primary circuit moves, in turn, the piston in the secondary circuit, whereupon its valve closes and a pressure can build up in it. The pressure increases in the system affects the pistons in the wheel brakes so that the vehicle is braked.
Springs in the master cylinder move the pistons back to their resting position when the vehicle is not braking. In this position, the valves are open and both chambers (brake circuits) are connected with the brake fluid reservoir.
The brake fluid reservoir for the master cylinder ensures that the correct amount of brake fluid is supplied in the system, irrespective of wear to brake pads and disc brakes.

Normal braking




The brake pedal is pressed and the piston nearest the power brake booster moves to the left. Pressure builds up in the primary circuit.
This pressure also moves the second piston to the left and pressure builds up in the secondary circuit.

Leakage in the primary circuit




The brake pedal is pressed and the piston nearest the power brake booster is moved, but does not build up pressure due to damage in the primary circuit. The piston moves until it comes into contact with the piston in the secondary circuit. Only then can pressure build up in the secondary circuit.
The brake pedal stroke is longer than during normal braking.

Leakage in the secondary circuit




The brake pedal is pressed and fluid in the primary circuit is pressed against the secondary circuit piston. The piston cannot build up pressure due to damage in the secondary circuit. It therefore moves to the left until it reaches the limit position in the master cylinder. Only then can pressure build up in the primary circuit.
The brake pedal stroke is longer than during normal braking.

Foot operated parking brake




The parking brake is mechanical and acts on the brake shoes in the brake drum on the rear wheels. The parking brake is applied by pressing the pedal on the floor. The handle, located to the left of the steering wheel below the dashboard, is used to release the parking brake.
The switch for the parking brake warning lamp is on the mechanism for the parking brake.
The foot operated mechanism does not have an integrated adjuster function, but is under the vehicle on the rear axle.
The parking pedal is on the left-hand side at floor height on left-hand drive cars. Right-hand drive cars and AWD vehicles have an electric foot brake.
The foot operated parking brake does not emit a clicking sound.

Drum brake function




The drum brakes are of the duo-servo type. Servo drum brakes mean that the friction force from the primary shoe is directly transferred to the secondary shoe when applied towards the drum. The combined friction force is absorbed by the armature in the same place that the expander is applied in. Duo-servo means that servo assistance is available at the front and back. Force is transferred between the shoes via an adjustable or fixed link. The expander is applied via a wire, which is controlled either by a hand brake lever or foot brake pedal.
The shoes are held in position in the drum brakes via a securing spring, which is fixed in the protective plate.

Parking brake, Park Brake Module (PBM)
For information, see Design and Function, Park Brake Module (PBM).