Many thanks for your prompt reply. My mast is, indeed, Selden. I visited their website but couldn't find any reference to damming the inside of the mast. If you can point me toward info on this I'd be much appreciative. Thx! Willy.
I'll get back to you tomorrow when I am at the shop.
They do indeed have a drain hole above the dam, but I can't for the life of me remember how they make the dam.
I can almost assure you that they do it with the mast horizontal though.