manual pump - shower drain question

[sandrunner]

I have a manual crank pump for the shower drain that goes out thru a seaock. New boat/new to manual pumps, but when I pump the shower to drain it, it spits up water each handle push and clicks on the handle lift action. It does draw down the water slowly, but rises a bit overnight to just below or at the the drain surface.

In the other head, I have the same setup, but it doesn't spit up water or click, thus wondering about the issue.

Any thoughts/suggestions would be much appreciated!