Like Bill R suggests... but also, look VERY closely at the smooth surface that the O-ring seals against. A scratch there can make it impossible to get a good seal. Anything you can feel with your fingernail is going to be a problem.
If you have scratches or other flaws, a very light addition of RTV gasket compound can fix it in the short run until a new smooth metal part can be obtained. Too much RTV here can really cause issues with the pump.
Charleston, SC, USA