Brake servo

The brake servo supports the force the driver exerts on the master brake cylinder when he/she presses the brake pedal. This significantly reduces the effort required when braking. Together with the master brake cylinder...

Function

Brake servo BoschThe brake servo supports the force the driver exerts on the master brake cylinder when he/she presses the brake pedal. This significantly reduces the effort required when braking. Together with the master brake cylinder, it is a component of most braking systems in cars. The brake servo only works when the engine is running. If the engine is switched off, e.g. when the vehicle is being towed, the brake force must be applied solely via the pedal.

Types of brake servo

The two most common designs are

Vacuum brake servo:

Most brake systems in cars have a vacuum brake servo. They use the vacuum that is produced in petrol engines by the air intake system in the engine's intake pipe or via a vacuum pump (0.5...0.9 bar) in diesel engines.

Hydraulic brake servo:

These types of servo use the pressure created by a hydraulic pump, which is driven by the engine. Hydraulic brake servos are used in

  • vehicles which have hydraulic energy supply (e.g. power steering) and
  • vehicles whose engine has low vacuum pressure in the intake pipe (e.g. turbo engines).

Hydraulic brake servos are smaller than vacuum brake servos and require a higher pilot pressure.

Function of membrane in the brake servo

There is a membrane inside the brake servo, which divides the servo into two chambers. When the brake is not being operated, there is a vacuum in both chambers, which is generated by the engine. When the brake is operated, both of these chambers are sealed off from one another. At the same time, a valve opens which allows atmospheric pressure to flow in on the pedal side.

Now there is atmospheric pressure on one side of the membrane (pedal side) and on the other side a vacuum (master cylinder side), which pulls the membrane connected to the push rod towards the master cylinder and augmenting the force from the pedal.

When the brake pedal is released, both chambers are reconnected with each other via a valve opening whilst the valve, which was previously allowing atmospheric pressure to flow in, now closes. There is now a vacuum in both chambers.

Safety

The brake is a safety-relevant part of the vehicle. A functioning brake servo helps the driver when braking. If it fails, the driver must then apply much more pressure to the brake pedal than he/she is used to when the brake servo is functioning. Therefore, should the driver notice a drop in braking effect, the vehicle should always be taken to a specialist garage.

Testing the functionality of the brake servo in four steps

It is generally possible to test if the brake servo is working properly using the following measures:

  1. Turn off the engine
  2. Press the brake pedal repeatedly until you feel a strong resistance ?(this means the vacuum still present in the system has been "used up")
  3. Keep the brake pedal pressed down 
  4. Start the engine. If the brake pedal now yields, the servo is working properly

This test is however no substitute for visiting a specialist garage as soon as the driver notices that the brake effect is dropping!

Depreciation

Brakes that function reliably are vital for safe travel in motor vehicles. They should therefore be checked regularly to ensure that they are in perfect working order and that their component parts are not suffering from wear. A well-maintained brake that is in perfect working order which a driver feels can be relied upon is the most important step on the way to feeling safe when driving.

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