Free fix
I got basted for posting this remedy before. Others will tell you that you have a crappy (pun?) head. But, there may be a simple and free remedy to get it going again. Emphasis on "free" and "simple" IE: no money(try $300 for "good" head), time, expertise, banged knuckles, cussing, driving, carrying, connecting, bolting, cutting, modifying, damaging, not fitting correctly...to do.
I do this about once every couple of months, usually when I've accidentally pumped all the water out of the intake line:
Open the thru hole. Place 3-4 small sheets of TP over the bottom of the head to prevent splashing up. Switch the wet\dry to wet. Now pump that handle like it's a Ferrari piston. I'm talking fast. The suction will draw the seawater back into the head, and the siphon will stay until you let the line go dry over time or if you pump it completely out.
Try it. If it fails, call me a moron. But, again, it's free and harmless to try.