Under the dash just about where the gas pedal is are three valves..at least in my 91 740...At idle listen for any hissing sound from this area which would suggest a leak due to a bad servo...in my case thats what it was...I had to remove the entire center console and then some to get to the valve. It was time consuming and despite the shortcuts as suggested in the faqs I would suggest removing whats necessary to do the job. I replaced the servo with a new one...cost about $65 and if my memory serves me right I got it from eEuroparts. By the way if you are able to remove the panel which covers the valves a simple test would be to kinda tug lightly on the shafts and listen for changes in the "hissing" sound if thats the case...hope this helps