Spongy Brake Pedal
There are several conditions that can result in a spongy brake pedal. It could be caused by loose wheel bearings or failing master cylinders internal cup seals. It's important to rule out different possible components.
Isolate the master cylinder by blocking the outlet ports with plugs and checking the pedal for firmness. If the pedal is firm the master cylinder cup seals are good and the problem exists downstream. The problem may be air trapped in the ABS unit, a line or hose, or a problem with air in one of the brake units. If you are able to clamp the hoses leading to the units and the pedal is still low, there may be air trapped in the hydraulic ABS unit. Inspect the brake hoses for cracks and dryness before clamping the pressure off. The hose may require replacement before proceeding with diagnostics. Inspect for leaks, repair and bleed the brake system according to manufacturer's specifications.
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.