I don't know the answer to that, so I did a little digging around. The only thing that I could find that seemed to make sense was to plug the output of the water pump and use a compressor to pressurize the pump from the inlet side. If it holds, you're good.
I don't know whether this will work with the pump on the sled or if it has to be off (or has to be on), or if there's more to it than that or not.
I also saw some information that says that you can get new bearings from NAPA for the pump for about $10. If you already have it half apart, you might want to finish tearing it down and rebuild it anyway (they're apparently known to have issues on that machine).