Marc's impact driver is the way to go but...What's with this "light" hammer tap stuff? Get a big azz screwdriver that fits the screw slot perfectly and whack the heck out of it one time with a big hammer. That's usually all it takes.Oh yeah, if you use an impact driver, make sure it's set to unscrew and not tighten it more. Some only go one way but other brands are reversible.
In my education through the school of Lessons Learned, and when dealing with marine stuff, I usually begin with the least power required to accomplish the task, then increase expotentially as the frustration level rises, usually ending in blood letting and destruction of the part, then Level II begins.
When you are filling it with the hand pump and the fluid starts to come out the top, walk away from it for ten minutes or so and let the fluid settle. When you come back to it, pump a little more fluid into it to top it off.