Low Spongy Brake Pedal
Aerated fluid results in a low spongy brake pedal. Bleed the air from the system to restore fluid pressure.
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. This internal leak causes the brake pedal to sink towards the floor slowly. Suspect leaking master cylinder cup seals if a low and fading brake pedal with no leaks found.
Loose and worn wheel bearings result in a low spongy brake pedal. Check wheel bearings by rocking the tire back and forth like the illustration. Excess movement indicates a worn wheel bearing. Rock the wheel from side to side to check the steering rack, suspension components, or tie rod ends for looseness.
Inspect each brake unit for leaks or swollen hoses. Have someone apply the brakes and watch each caliper assembly for an excessive twist: loose slide pins or mounting result in a twist and inefficient braking force at that unit.