Heavy goods vehicles and buses - Category C and D.

3 - Brakes

Cars are equipped with brakes in order to stop the vehicle safely, quickly and effectively. Brakes are therefore a very important element of safety, and it is therefore important to have sufficient knowledge of the braking system, its operation and maintenance to ensure that the brakes are working properly and that any faults are detected in time.

The wheel brake consists of a stationary part that carries brake shoes or brake pads and a rotating part, which can be either a brake drum or a brake disk. The drum brake basically consists of a set of brake shoes that can be mechanically or hydraulically pressed against the brake drum.

Air brakes

The supply circuit

Compressor
The compressor is a pump that sucks air from the outside through a filter and sends it to the supply circuit as compressed air. The compressor is driven by the engine of the vehicle.

Brake circuit (compressed air mechanical brakes)

The brake circuit is the part of the braking system that is pressurized when the brake pedal is applied, thereby activating the wheel brakes.

The brake circuit consists of the following main parts:

• Brake pedal/brake valve.
• Brake force regulator. (ALB valve).
• Brake diaphragms.
• Pipes and hoses.
• Wheel brakes.

If the braking system is designed as ABS, in some cases there will be no brake controller. If ABS does not work, continued driving is illegal.

Brake pedal

When the brake pedal (and thus the brake valve) is operated, compressed air is directed from the supply circuit to the brake circuit. The further the pedal is depressed, the greater the pressure in the brake circuit. If there is no compressed air, the brakes will be ineffective.

The brake valve is checked as follows:
Must have a small clearance in the top position. It must be possible to depress the brake pedal to a firm stop so that the brake valve opens fully.

Brake force regulation (ALB valve)

1. ALB valve.
2. Oscillation damper.

The brake force regulator (ALB regulator) automatically adapts the braking force to the vehicle's load. The ALB controller is usually mounted on the chassis frame and is in mechanical connection with the wheel axle.

Checking the ALB regulator:
The mechanical connections of the ALB regulator must be intact and the valve arm must move freely. The function of the regulator is checked with the service brake activated and by moving the valve arm up and down. This should change the air pressure in the brake circuit, which may be heard when air leaks from the brake circuit. However, this check cannot be performed on all types.

ALB with air suspension
On vehicles with air suspension, where the distance between the chassis frame and the axle is the same regardless of the load, the brake force regulator is controlled by the pressure in the air bellows. The air pressure in the air bellows increases with higher loads. When the bellows pressure changes, the control pressure at the air-controlled ALB valve, which regulates the compressed air to the brake diaphragms, also changes.

Signs of faults:
Reduced braking effect indicates, among other things, incorrectly adjusted or defective ALB valve. Rear wheel lock-up during moderate braking may indicate an incorrectly adjusted or defective ALB valve.

Air operated ALB valve

Brake diaphragms

Brake diaphragms, located near the wheels, are responsible for converting the energy of compressed air into motion and force to affect the wheel brake.

The piston travel of the brake diaphragm must not exceed a quarter of the diaphragm housing clamping band diameter.

Brake adjustment

As the brake lining wears, the stroke increases. Braking performance is reduced or fails completely when the stroke becomes too long. Brakes must then be adjusted, which is done by adjusting the brake key lever. Newer vehicles (1996) have automatic brake adjustment. It is important that function and stroke length are checked and adjusted regularly. For heavy braking, the check should be done frequently.

Self-adjustment of brakes

Self-adjusting brakes are often equipped with an indicator ("telltale") that indicates the amount of brake lining remaining. With the "indicator arrow" standing as the mark (1) full brake lining quantity. When the "indicator arrow" (1) is horizontal upwards and next to the mark (2), it shows worn brake lining.

Signs of failure:
Extended operating time or excessive consumption of compressed air and thus excessive pressure drop when braking can be a sign of excessive piston travel in compressed air diaphragms or cylinders. Excessive pressure drop can also be caused by water in the compressed air reservoirs. During full braking (with the brake pedal fully depressed), the pressure drop should generally not exceed 0.5 bar.

Pipes and hoses
Pipes and hoses connect the individual parts of the brake circuit.

Signs of failure:
Pipes and hoses must be intact and free of corrosion, cracks or leaks.

Tightness test/load test
Check that the tightness and load of compressed air-mechanical and compressed air-hydraulic brakes are met in both the supply circuit and the brake circuit, as assessed by the following test:

Leak test is performed by bringing the pressure in the supply circuit down to the lowest working pressure. With the brake pedal in the down position, start the engine and bring the pressure up to the highest working pressure, stop the engine, keep the brake pedal depressed and listen for leakage.

If this can be done without a sudden pressure drop (read on any pressure gauge) and without audible leaks after stopping the engine, the system can withstand sufficient load.

Working pressure:
The system must have the required working pressure (according to the instructions of the vehicle manufacturer) before driving begins.

Compressed air-hydraulic brakes

Transformers

Compressed air-hydraulic braking system consists of a compressed air part (supply circuit and part of the brake circuit) and a hydraulic part. The brake pedal (and thus the brake valve) regulates the air pressure to a compressed air cylinder that affects the piston in a hydraulic master cylinder. This converts the air pressure into hydraulic pressure.
The hydraulic pressure is routed through pipes and hoses to the wheel brakes. The air cylinder and the hydraulic master cylinder are built together and are called the transformer. If there is no compressed air or insufficient brake fluid, the brakes will be ineffective.

Compressed air-hydraulic brakes are checked as follows:
The brake pedal must be able to be depressed to a firm stop to fully open the brake valve. Must have a small amount of free play in the top position.

Pipes and hoses must be intact, secure and free from corrosion, cracks or leaks, this is checked on readily accessible parts. Mechanical brake force regulator (ALB valve)
Must have an intact mechanical connection. Depending on type, the valve arm must be able to move freely. As a general rule, the air consumption during full braking must not exceed 0.5 bar, as this may indicate a lack of adjustment of the brakes, resulting in longer piston travel.

ALB valve (hydraulic)

1. Measuring stick

Brake position.

The piston travel of the transformer must not exceed the limits specified in the instruction manual, judged by the travel of the dipstick or by warning lights. The brake fluid reservoir of the master cylinder must have a fluid level between the minimum and maximum marks. If the fluid level is too low, the warning light on the instrument panel must come on.

Signs of failure:
Extended operating time or high compressed air consumption can be a sign of too much piston travel in the transformer. Oil contamination of the anchor plate or the inside of the tire may indicate a leak in a hydraulic wheel cylinder.

Tightness test/load test
The compressed air system must be sufficiently tight and withstand sufficient load in both the supply circuit and the brake circuit and be checked as for compressed air mechanical systems.

Combination of braking systems:

• The braking system can (most commonly on buses) be a combination of air-mechanical and air-hydraulic brakes.

Hydraulic brake with vacuum reinforcement

The service brake on lighter vehicles may be supplemented with a vacuum brake servo, which amplifies the hydraulic pressure when the driver steps on the brake pedal. Vacuum is supplied either from the intake manifold of petrol engines or from a pump in diesel engines. In the absence of vacuum, the vehicle can be braked by brake pedal pressure alone, but braking will be less and continued driving will be dangerous and illegal.

ALB valve

The brake servo is checked as follows:
With the engine stopped, depress the brake pedal a few times to "remove" the vacuum from the brake servo. Then keep the brake pedal depressed while the engine is started. The pedal should now sink a little if it does not sink, there is a fault in the brake servo system.

Signs of failure:
Long pedal travel in a hydraulic system with a servo can be a sign of brake misalignment. A spongy brake pedal in a hydraulic system with a servo can be a sign of air in the hydraulic system, among other things.

On smaller buses (max. 3500 kg.) with hydraulic brakes, the service brake will in practice have such an effect that the braking distance at 30 km/h. under all load conditions does not exceed approx. 6 m.

1. release 2. braking 3. Prohibited

Emergency brake - ABS system - Parking brake

1. Diaphragm cylinder 2. Sensor 3. control valve 4. brake valve 5. controller (microcomputer)

Emergency brake
The purpose of the emergency brake is to be able to brake the vehicle in the event of a service brake failure. The emergency brake is either included in the service brake or the parking brake.

Anti-lock brakes (ABS system)
Design and effect of anti-lock brakes (ABS brakes). All types of braking systems can be designed as ABS brakes. The ABS system is designed in such a way that sensors are placed at each wheel that detect the rotation of the wheel by means of a toothed rim. The sensors send signals about the rotation of each wheel to an electronic control box. If one or more wheels tend to lock during braking, the electronic control box will regulate the pressure to the wheel brakes to prevent the wheels from locking. The ABS braking system is designed to enable steering and braking simultaneously to a certain extent. When braking a heavy goods vehicle with ABS brakes, the ABS braking system ensures that the wheels are kept in rotation and do not lock. When the ABS braking system regulates, some smaller vehicles may experience vibrations in the brake pedal. This is normal and means that the system is working. Regardless of the vibrations, the pressure on the brake pedal should be maintained as long as braking is desired. If the ABS braking system is not working, the braking characteristics will change and only driving to the nearest service garage is allowed.

Spring brake (parking brake)
The spring brake is triggered by compressed air pressing a piston against a compressed spring. When the parking brake lever is activated, the compressed air is removed and the spring force is sufficient enough to activate the wheel brakes.

How it works
When the handle is turned the first time, the emergency brake (the spring brake of the vehicle and the service brake of the trailer) is activated. The braking force is regulated according to the distance the handle is turned.

To determine whether the parking brake alone can brake the vehicle combination if the air pressure of the trailer disappears, turn the lever further to the control position (the braking of the trailer will cease and the vehicle combination is only braked by the spring brake of the towing vehicle).

Checking the parking brake:
When activating the spring brake to the braking position, you should hear the compressed air escaping from the cylinder.

It must not be possible to pull the handbrake lever to the bottom position for other types of parking brakes.

Other legal regulations regarding brakes

• The vehicle must be equipped with a dual-circuit service brake, emergency brake and parking brake. Newer vehicles must be equipped with ABS brakes.
• The service brake must act on all wheels and be able to brake and stop the vehicle safely, quickly and effectively at any speed and under all loads. It must be possible to achieve the required effect of the service brake at the first application of the brake pedal.
• On systems without pressure gauges, the compressor must be able to fill empty compressed air tanks in less than 3 minutes until the warning light goes out or the alarm is deactivated. On systems with a pressure gauge, 2/3 (65%) of the working pressure must be reached in less than 3 minutes.
• The parking brake must be able to keep the vehicle stopped on an 18% incline and remain applied.
• The emergency brake must be able to brake and stop the vehicle safely and effectively in the event of a service brake failure.

When driving on a horizontal, dry road with an asphalt surface, check if the following requirements are met:

The maximum braking distance at 30 km/h for heavy goods vehicles and buses is approximately 10 meters.

• When slowly depressing the brake pedal, the braking effect must increase steadily (judged by a practical brake test).

The braking distance should look like this:

1. The brake pedal is depressed 2. The distance the vehicle travels during the operating time 3. The distance used for braking