If your vehicle is showing brake pedal feels soft or spongy, here are the most likely causes and the parts you'll need to fix each one.
Air is compressible, unlike brake fluid. If air enters the hydraulic system during recent brake work or due to a low fluid level, the pedal will feel immediately spongy.
Brake fluid naturally absorbs moisture over time. Under heavy braking, this moisture boils into a gas, which compresses and causes a soft pedal.
Internal seals inside the master cylinder can wear out, allowing fluid to bypass the piston. The pedal will feel spongy and often slowly sink to the floor under steady pressure.
A leak in a brake line, hose, or caliper reduces hydraulic pressure and introduces air. This is usually accompanied by a dropping fluid level in the reservoir.
Old, degraded rubber brake hoses can weaken internally. When you press the pedal, the hose balloons outward instead of directing all hydraulic pressure to the brake calipers.
On vehicles equipped with rear drum brakes, if the self-adjusting mechanism fails, the brake shoes have to travel further to contact the drum, resulting in excessive, soft pedal travel.