Brake diagnosis help needed

["Steve Shoyer" <Steve@shoyer.com>]

My brake pedal will slowly sink to the floor with constant pressure.  The car 
is a 1980 MGB.  The reservior is full of fluid and doesn't appear to be leaking 
or losing fluid, the brakes have been bled repeatedly, and many of the 
components have been replaced or adjusted.  I'm guessing that the brake master 
cylinder needs to be replaced (so nice of Moss to have them on sale now!), but 
is there a way to verify that this is the problem?

Thanks.

--Steve Shoyer (1980 MGB)