My s280 w140 idles at 750 rpm when in park or neutral. This is fine, but when i engage drive the revs drop to 500 which causes problems. When i stop on a hill the car does not stay still but starts to roll backwards and i have to feather the throttle. I have been told that it is the vacum modulator on the gearbox, can anyone give me some advice as to what might be causing the problem.
Many thanks
Lester.