Yes like Texasmark1 said, a spongy pedal is a sign of air in the system. I would bleed the brakes starting with the right rear, left rear, right front then left front. A bad master-cylinder usually will let the pedal bottom out only with light constant pressure applied to the pedal. This is called Master-Cylinder Bypass. The fluid bypasses past the rubber cups inside the Master Cylinder.
If you install a new master cylinder or have some one else do it have them change the brake fluid in all the lines. Some companies recommend changing the fluid once a year.