It's hard to tell without looking. It could be either a shaft seal -- you have to be really careful not to scratch the carbon face,or get grit between the carbon and the ceramic during assembly, or the gasket whether the carbon seal goes on the motor shaft (slid on smoothly, no shaft roughness or dirt -- OR some of the pump rubber.

It's good practice, when rebuilding a pump to replace the seal AND all the casing rubber -- O-rings, gaskets, etc. Use OEM rubber too. I've always gotten unpredictable results from aftermarket rubber parts.