My car was stored for two years, and while on a long shake-down drive yesterday (high 80s weather), suddenly the brake pedal began sinking while stopped at intersections.  A few pumps always brought back a hard pedal, but obviously I figured the ride was best cut short, and soon turned tail for home.  I had a modern jap driver with anti-lock a few years back that exhibited the same symptoms, until the brakes gradually failed totally.  A new master cyl. cured that issue, but I am not certain how the complexities of a modern dual master system might translate to my '55 OEM part (replaced 17 years ago).  Sitting in a cool garage now, the pedal is hard, and will not sink with constant pressure. But, due to the T-Bird master's proximity to the exhaust manifold, only another test drive will indicate if extreme heat is a contributing factor.  While I DO plan to pull the drums and inspect all wheel cylinders, banjo connectors, lines, etc. for leaks, I notice the firewall pad/carpet is dry, and the brake fluid level is unchanged.  The fluid IS nearly 4 yrs. old, so upgrading any worn components and bleeding (I have a pressurized canister system) won't constitute much inconvenience.  Regardless, I really know very little about the mechanical construction & operational details of master cylinders, not to mention installation prep. tips, etc., and am just wondering if anyone out there with experience/knowledge could offer any pertinent advice?  

6 VOLTS/POS. GRD. NW INDIANA