I tend to believe the the problem is that I cannot get the extractor tube in the deepest part of the oil resevoir. So, the question is: how do I get it deeper. Is my particular model (88' 420SEL) of mercedes the problem? If I had a cross section of the oil pan and the dip stick tube I could see what the restriction is. Is there such a cross section view available? Or does anybody have such a view who can explain what may be obstructing the entry of the extractor tube? Apparantly nobody else has had this problem.
Sixto, why didn't I think of checking the extractor pressure holding. Guess I'll use it on a bucket of water and see how it works.