Low Spongy Brake Pedal

The master cylinder pushrod adjusts brake pedal height. It's usually (almost always) made of two parts and lengthened or shortened by adjusting a nut located on the rod. This adjusted nut is then locked in place with a locking nut located on the same rod. Under-adjustment results in excessive pedal free play and a low brake pedal, and over-adjustment can lead to wheel lockup.

Master cylinder.

Contaminated, overheated, or aerated brake fluid also causes a low spongy brake pedal. Brake fluid is hygroscopic, meaning that it has an affinity to and attracts moisture. If a container of brake fluid is left open, it absorbs moisture and quickly becomes contaminated. Always keep the brake fluid container sealed and stored away safely.

DOT3, DOT4, and DOT5 brake fluid,

The DOT Department of Transportation rates brake fluid. They assign them a DOT rating, DOT3, DOT4, and DOT5. The higher the DOT rating, the higher the boiling point. Moisture in a brake system lowers the boiling point. If moisture is suspected, a brake flush with the correct DOT fluid is required. DOT5 brake fluid is silicon-based and DOT3, and DOT4 fluids are glycol-based. Never mix the two; it results in a spongy pedal and a loss of braking performance.