Aside from an actual leak in the top plug, which you can easily check by removing the mast head. Water can get in the older CompTips via the mold part line. I would drill out one of the halyard lock rivets and see if the water will drain there. If not, you may have to poke a wire down through the foam plug material. Re-rivet the hook with a rivet cap included to seal it. Otherwise you could drill another hole somewhere and re-seal with a rivet and cap. Then paint the mold part line with some resin. There is also a mold part line under the luff track, but it is less likely to leak much from there. The part lines have been painted over on newer boats for years. It was discovered that the glass structure could allow moisture through that would condensate inside the tip and by repeating the process (cooling and heating) the tip, it would end up with quite allot of water inside. Possibly do a leak (air pressure and soapy water) test if you drill a hole to drain it. That would help expose the actual leak point(s).
