FWIW ....
I have a Shurflo pump in my Motorhome and it works great.... Except ..... it would never prime the tank after it ran dry and I had fiddle around for ages opening the outlet and messing around with trying to eliminate airlocks. But once water primed, not a hint of an issue.
removed the pump and tested it with a
battery and bucket of water and never any problem on testing.
I spoked to the previous owner after I got fed up with this to see if he had any ideas (not to complain, but he could tell me what he used to do).... He had the same problem and basically he just used to make sure the tank never went dry and on the odd occasion he did he messed around with the pump outlet until it got going again. He said he replaced the pump also to no effect.
After two years of this, I had enough and decided to get another pump just to see.... Got a new Shurflo pump ( like this, but the 30PSI version -
https://amzn.to/40fJ78b), fitted it and never had a second of problem since. Primes inside a minute with no intervention.
My takeaway from this was even if something appears to be in working order, and when put on a bench test it tests good, there can still be something wrong but impossible to find.
Life is too short to be lying down with your head in a cupboard trying to get something that apparently works to actually work!
My only regret is not just buying a new pump sooner. Kev ... just get a new bloody pump and have done with!!