You can get rebuild kits or have them rebuilt. contact will Samples who does both. But, first, as suggested check all vacuum lines and hoses for leaks. Perhaps simply replace all rubber sections of the vacuum lines as a precaution, then see what gives. Also, might the rubber sections of the fuel lines be so old that they allow small amounts of air to be sucket into the fuel system beginning at the tank then going forward?

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Douglas
1959 M-B 220S cabriolet