In my case it was the Piston
My pump (Jabsco) became very stiff after just a couple of weeks despite liberal coatings of silicone grease. Curious I measured the piston (ugh!) and the pump cylinder bore and found that with the "O" ring in the bottom of its groove in the piston it was then an interference fit in the pump bore. So, when the lube had been squeezed away it could and would bind. It hardly needs to seal like this as the pump motion seals it against the top and bottom surfaces of the groove as it goes up and down and the contact with the inside of the bore does the rest. I just made the groove a bit deeper using a file. It only needed a quick go with a flat file being VERY careful not to scratch top and bottom surfaces of the groove. I put self adhesive tape on each side of the file and just used the exposed edge in the base of the groove. I reduced the groove diameter by only about .015". The "O" ring does not need to seal to the inside of the groove as explained above. Since then no need to lubricate it at all - AND it still primes from dry no trouble. I guess Jabsco got it wrong!
