RPM flucuates when pressing brake pedal, normal otherwise.
1 Answer
The power brake booster works by receiving (suctioning) vacuum (air) from the engine, as the brake is applied and air drawn from the engine, the revolutions will go down/up to compensate, this will create the RPM fluctuation that you see. Depending on the size of the engine, this will be more/less noticeable in different vehicles. My 2005 Ford Five Hundred has this slight variation, while it is not noticeable in my 2017 Mercedes-Benz E300. However, if you notice that the fluctuation is very strong, have the brake system checked, even if the "check brake" light does not come on.