I changed out the fuel filter in my 1981 300D this weekend. When I tried to use the primer pump it didn't seem to work. I was under the impression that you unscrewed it slightly and then pumped it. I unscrewed it as far as it would turn and it still would not pump. Am I doing it incorrectly or is this a sign that the primer pump is bad? By the way it appears to be the old style pump, but the top is metal rather than plastic like the pictures I have seen. Maybe I will just swap it out for the new style pump.