See pix here:
http://tinyurl.com/jgsmp
The pylons are sections of H-14(?) mast extrusion, so they are hollow and the "seam" on the back is the luff track. Each pylon has a closed cell foam plug....the forward pylons have small vent tubes through the foam. The inside of the corner castings are not open to the inside of the crossbars.
It amazes me that so much water gets into most H-16 hulls. Sometimes I think rain water must get in somehow...or maybe it's condensation. Anyway, your foam plugs are probably shot, but I don't think it would be worth the effort to fix them with the amount of leak you're describing. On my previous boat, I replaced my pylon plugs with Great Stuff when I had the boat apart to epoxy the castings, do a bottom job, etc. in a last ditch effort to keep her racing. For now, my advice to you would be to drain after every sail and (everyone should) leave the plugs out to let her breathe while on the trailer. Just don't forget to put them back in for sailing! Hope this helps!
Jerome Vaughan
Hobie 16
Clinton, Mi'sippi