Woodside Auto services

My 2006 ,2.7 diesel S Type has a brake problem, simply put when you gently press the pedal it almost goes down on its own,could it just be a weak spring in the servo or is it possible to have too much vacuum on it from pump, also is it possible to fit a servo of an xf,as the part number on mine matches an xf,but doesn't appear to match any I've seen or enquired about ,ps the brake's work fine it hasn't got a soft pedal.thanks for any help