Spongy Brake Pedal
A low spongy brake pedal is usually caused by air in the system. Bleeding the system will remedy the situation. Master cylinders have seals that build pressure when the pedal is depressed. These seals wear in time and fluid pressure begins to bypass the seal causing the pedal to sink towards the floor. This is a common problem when there’s a low and fading brake pedal with no leaks found.
The wheels should be checked for loose wheel bearings. Loose or faulty wheel bearings will result in a low spongy brake pedal. Never assume it's some other component like the master cylinder without checking other possible causes. Check these bearings by rocking the tire back and forth like the illustration. Excess movement indicates a worn wheel bearing. If the tire rocks from side to side, the steering rack, suspension components, or tie rod end looseness (most likely) should be inspected.
Inspect each brake unit for leaks or swollen hoses. Have someone apply the brakes and watch each caliper assembly for excessive twist. Loose pins or mounting will result in twist and inefficient braking force at that unit.