Mast Leakage Causes
My two cents... I recently reworked my Cat 30' SR mast step and have read quite a bit about similar problems other people have had. If you removed your mast and the mast step (plate on the cabin top under the mast), you will find only a hole a bit larger than 1" in diameter. Typically, a ~4" piece of 1" diameter PVC pipe is installed here so that most of its length extends above the cabin top. The piece of PVC pipe is caulked into place with VERY generous amounts of sealant and the aluminum mast step (plate) is bolted in place on top of this. All of the mast wiring and cables are pulled through the PVC pipe an up into the mast. The only way water can get into the interior of the boat is if: 1) the sealant isn't sealing around the PVC pipe or the PVC pipe has been dislodged or 2) if the mast isn't draining properly at the weep holes on each side of the mast step plate and the water fills up the mast beyond the top of the PVC pipe. Since you have already checked that the mast is draining properly, the only solution is to unstep the mast and rework the PVC pipe/caulking/base plate installation to ensure there are no leaks. While this is indeed a hassle, it is nothing near as bad as having to deal with mast step compression when the leaking water permeates and damages the mast step. Good luck!