Could be either the pump seal, or the pump body O-ring.

=> It's very easy to damage a pump seal when installing it. Also, running it dry, even for just a minute or two, can damage it.

=> Large body O-rings like that are usually a beast to get back into place. Pinch the O-ring and it leaks. Get a bit of grit on the O-ring and it leaks.

If you look carefully, you may be able to tell whether it's leaking at the seal, or at the body O-ring.

Good luck!