I think the comments made subsequent to your first message were pertinent
to you. On our Delta Plus, when the small 1.5 liter (I think they were)
pumps went out, I replaced them with larger pumps, similar to the rough
pump backing the source.

I also have had bad experience rebuilding those small pumps, and after
throwing money down a pump "rat hole" decided they were not worth
repairing. I pitch them out when they go.

Marilyn Fogel