Two possibilities as I see it, the Joker valve and or the intake valve, Pardon me for not knowing what it's called but a pump must have both an intake and an exhaust
valve to function.
I'm going to bet that pumping out the tank put a vacuum on the flapper intake valve and un-seated it, I have the exact same thing on my boat and it started this weekend when I put a new pump unit on the head
. I do not have the same brand head
, but assume operating principles are the same.
Or you have a clogged vent, vent is easiest to check, I would start there