I have a vacuum leak in my 1981 300D. I have narrowed it down to either of the two back doors and /or fuel door. I am testing via the lines under the front passenger seat. Do you need to test with the drivers lock up and then down? One of the doors will hold vacuum on one side(locking) but not when unlocking. Does that mean it is bad? The other door won't hold vacuum either side. The rear line will hold when you are locking the fuel door and trunk but not when you are unlocking, at least not the fuel door. ALso, when I am checking the trunk and fuel door, if I lock it with vac pump when I disconnect the trunk stays locked and the fuel door goes back to unlock. Should the fuel door stay locked?
thanks. Rob Thomas

At dealer WE would suggest that 20 year old rubber diaphrams wear out at the same rate on these cars & that you should replace ALL of the elements as repairing 1 will cause strain on all of the remaining ones, & 3 trips for diagnosis will make any one angry! There is pressure or vacuum on this system at all times!