I am going to assume that you have double-checked all of the brake bleeder valves to make sure that they are closed tightly and not seeping brake fluid. I am also going to assume that you have also checked all of the brake lines for signs of damage and for signs of leaks and also verified that all of the connections to the Master Cylinder are tight.
The mushiness in the brake pedal is one of the signs of a failing Master Cylinder. Some of the other signs are the brake pedal slowly going to the floor when the brakes are held on. Having to "pump up" the brakes before stopping is also a symptom. The signs of moisture around the Master Cylinder is also another symptom.
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