Thanks for your help CD!
Its a hunter 310 (1997). I have tried to spray the MS with a hose while watching below, and couldn't get the leak going. I'm not sure if this was because we were only spraying the MS foir a few minutes (like I said, the leak only occurs after long, heavy rains). If the water is coming through inside the mast, does the mast have come down? I have never seen the sealing system inside a mast step, what is going on in there?