The way in which the servobrake obtains a vacuum differs between petrol vehicles and diesel vehicles.
In a petrol engine, the vacuum is obtained through a direct connection between the brake servo and the intake manifold, with only one non-return valve along the line.
For diesel engine vehicles, a vacuum pump operated by the engine is required.
The difference between both systems lies in the capacity to intake air (air flow in litres per min) and the level of pressure provided by each system.
In the petrol engine, the flow is greater. This means that the time required to drain the volume in the brake servo is small. On the other hand, the vacuum pump used in diesel engines requires more time to drain the same volume of air, however, the vacuum pump is able to reach higher vacuum levels.
This means that following two or three consecutive brake applications for a diesel vehicle, if the brake pedal is held, an unwanted movement of the pedal occurs without increasing the foot force. This is due to the fact that after the brake has been applied several times, a large volume of the vacuum is exhausted and the vacuum pump needs a few seconds to replace it. This pedal movement is a sign that the system is resetting itself .
This situation can be felt when the car is at a standstill, and the driver has no feeling of deceleration, because in fact when the pedal is depressed as far as possible the driver is creating a braking pressure 4 or 5 times greater than that required.
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