How to Diagnose a Low 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 cylinder 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.