[flybouy]

That spring is a check valve that holds the water in the pipe when the pump is used. By removing it, if you don't have a hose on it the water pump will pump the water out of the fitting. Oftentimes they can stick and if you carefully push it in with a screwdriver you can free it up. Usually there's a rubber washer with a dome shaped screen in the hose fitting. If it's missing, or put in backwards then the screen won't push on the plunger when the hose is screwed on.