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It is very hard to say why the pump is not pumping on the engine. The first thing that comes to mind is that the pushrod is not operating the diaphragm. As you say it is pushing out a 1/2 inch when turning the engine. It should work fine with the pushrod moving that distance. Perhaps it is sucking air. Or some dirt in the fuel are now causing one of the valves not to close. As you state that it was pumping fuel for 20 minutes.
It is all I can think of other than fuel leaks and or dirt in the valves there cannot realy be a fault.
When you disconnect the fuel pipe from the carb, is there fuel pressure?