Given the Hypotenuse of 100 feet and the angle of 45 degrees, I calculate the rise to be 70 feet. At .43 PSI per foot, you are losing 30 PSI at the cabin. I would suggest that you change out that jet pump for a submersible.
Anyway... that was not the advice you came here for. To prime the pump after replacing the footvalve, I suggest you pour the water into the pipe up at the cabin and bleed off any air at the hose bib near the pump.