I just rebuilt my 1973 Holley 1920 using Walker kit 15519A. According to the factory service manual, page 14-13, has instructions for reinstalling the choke vacuum diaphragm. The instructions say to determine if the diaphragm has internal leaks to depress the diaphragm stem and then place a finger over the vacuum fitting to seal the opening. Release the diaphragm stem and if the stem moves more than 1/16th inch in ten seconds, the leakage is excessive and the assembly must be replaced. I did this test and the stem moved more then 1/16th inch very quickly. My rebuild kit did not come with a choke vacuum diaphragm. Can they still be bought?

Yep, they can still be had new - if you strike out locally, send me a PM; I have several new ones on the shelf.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

If you have an Auto Zone close by, try there. That's where I found one for my Carter BBD 2-bbl.

Roger