Sinking brake pedal when operated while stationary and with the engine running

 

(Normally with diesel vehicles)

Frequently after repair or service work on the brake system there are customer complaints about the sinking brake pedal when operating the brake with the vehicle at standstill and the engine running. This is a frequent complaint with diesel vehicles. This symptom is attributed to the different functions of suction pipe and vacuum pump.

Prerequisite:

Always makes sure first of all that the brake system functions perfectly and that there is no air in the brake system.

Explanation and effect:

By pressing the brake pedal several times in short sequential intervals while running the engine with idle speed the vacuum quantity in the booster will be reduced. With increasing brake pedal operation brake boosting is reduced, the brake pedal pressure becomes hard. If the driver's foot remains on the brake pedal and the vacuum in the booster increases as a result of the running engine, then the brake boosting will increase again. This results in the brake pedal sinking. Now the maximum vacuum capacity has built up again in the booster. With diesel vehicles and also with the newer petrol engines (direct fuel injection) the vacuum is created by an additional vacuum pump, as described above. Based on this the brake pedal on these vehicles drops slower and is more easily noticed by the driver. This sensation is frequently interpreted as a fault in the brake system.

After releasing and reapplying the brake pedal the pedal feel will be normal again. This process is purely physical. There is no fault in the brake system. The same sinking of the brake pedal is noticeable with the brake booster test.

 

 


 

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