u mean the "coolant reservoir"? Not the water pump right? U dont fill a waterpump.

If its draining your reservoir, you might have a leak, did you check for leaks? Let it run idle on a clean part of pavement in a parking lot somewhere. Lie on the ground with someone "observing" the temp so that doesnt go high. See if there are any drips of any kind on a consistant basis.

Dave