Navigation-bar problem^^^please help

hi, i have a big problem with my navbar!!!
i want that under_mc(the movieclip under the navbar) get an alpha of 0.5 when i release the navbar(content_mc).
it works as far i only have 1 button, but when i add some more buttons will it allways reuse the get transparent code

here`s the link to the fla.::

thx in advance!!!