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Re-install the diaphragm and test the actuator again.
After cleaning and greasing the diaphragm held a solid 16Hg vacuum for several minutes with no change. There must have been some debris under the edge of the diaphragm letting air in slowly. Now that the leak has been fixed it can be re-installed.
If cleaning and greasing the diaphragm does not stop the leaks, or if the diaphragm is visibly cracked or torn, the actuator will need to be replaced with a new or good/tested used actuator.