I haven't done it, but found this link:
http://www.carbkitsource.com/carbs/tech/Zenith/Zenith%20Model%2028%20228.html"One of the more difficult parts of restoring these carburetors is the removal of the pump discharge check valve (#38), found in the bottom of the accelerator pump bore. This can be removed by screwing a wood screw into the brass check valve and pulling out with a slide hammer. Re-installation should be done with a suitable sized drift so that the valve can be installed square and without damage."
