Sponginess alone is usually not caused by a bad master cylinder. If you don't have visible leakage caused by deteriorating grommets or corroded fittings, the other failure mode would be leaking piston seals due to wear or corrosion. This will cause fluid to leak back into the reservoir, and you will notice that your pedal continues to sink to the floor to compensate.
Sponginess = gas in the fluid or elasticity in the brake lines
Sinkiness (yes,
approved King Julien word) = fluid leakage around internal seals or out of fittings.