When I first got my old Stealth there was water already in the mast and I couldn't seem to get it out.... Obviously it had got in somewhere? The approach I used was to drill a hole at the side of the mast just above the foot plug and empty it (from memory it took about 3-4 mins to empty through an 8mm hole). Next step was to epoxy a Schrader car tyre valve that I cut from an old inner tube into the hole.
Once this was dry I connected a foot pump to the valve and raised the pressure VERY VERY SLIGHTLY (about 3 pumps). Then, using soapy water, I checked around all of the fittings and finally along the length of the mast. I fount that both my upper diamond attachment points were leaking and one spreader attachment. I drilled off each fitting, resealed with sikaflex and reattached the fittings. After checking again for leaks, I removed the schrader valve and epoxied over the hole. Job done!

With hindsight I think I'd have left the valve in place but made a neater job of the install......