I had this problem with my mast and did the following:

I drilled out one of the four rivets on the mast base and replaced it with a self-tapping screw and o-ring. Once the mast was drained, I took the mast top off, replaced the internal 'dam' and used sikaflex (do not use 5200, as you will never get rid of it)....

Now, when, and if there is ever water in the mast again, I can simply remove the screw and let the mast drain overnight.

Hope this helps...