I have a short PVC pipe that runs through the center of the mast step (kind of towards the front of the center of the mast step). The cables for the antenna and mast light are suposed to run through the pipe. The pipe itself is plugged with silicone caulk around the wires. The pipe in the center of the mast step should be a little above the deck level so the water is forced out the weep holes in the mast step. I would check to see if the weep holes in the mast step are cloged. If they are clogged water can run down the inside of the mast, fill the mast step and overflow the tube and then down the compression post. You can use a short piece of stiff wire to clean them out. Aso check to see if the calk in the pipe hasn't become dislodged. If is has clean it out and replace it. (I haven 't tried it but I've heard you can use electricians putty or maybe plumbers putty instead of caulk to plug the pipe instead of caulk.) It's also possible that the pipe whas been pushed down and is no longer above the deck. If it is push it back up and recalk inside the mast step.
Hope this helps.....