One other possibility that I have experienced when pump runs continuously and very little water comes out. The rubber valve inside the pump may look perfectly good, but can be worn out. When I called Shur-Flo about it after taking the pump apart and not seeing anything wrong, they said I needed a new valve, which would be cheaper than a new pump. I was a little skeptical, as the old valve looked good, but ordered a new one anyway. Guess what - the manufacturer knew their stuff - the new valve solved the problem.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.