For what it's worth -
I just rebuilt my Whale Gusher Titan manual pump. The flapper valves had
lost flexibility (like me

) and were not opening and closing sufficiently to get a suction going from the bilge out through the pump. Original pump - 18 years old.
I disconnected the lines from both sides and put a
water hose in each of them to ensure that the hoses from the pump to the bilge and out through the
thru hull were both clear. I ordered a couple of
rebuild kits and installed one and now it's working fine.