A brake pedal that is functioning optimally has a tight hold on the brakes, so it should feel firm. A soft brake pedal is when the brake pedal does not have that firmness.

Causes of a Soft Brake Pedal

Air in the brake line(s)

Your vehicle's brakes rely on a hydraulic pressure system to operate, and brake fluid helps to keep that pressure. Brake fluid is the only component that should be in the brake lines. If air gets into the lines, it can prevent the fluid from flowing properly, causing the brake pedal to feel soft.

Damaged brake line(s)

Since the brake lines are made of steel tubing, they can become rusted. Over time, the rust can develop small holes and collapse the inside of the hose into the brake lines, allowing brake fluid to leak out of the lines. The loss of brake fluid leads to a loss of hydraulic pressure and the brake pedal will feel soft.

Bad disc brake calipers

Like brake lines, disc brake calipers can also corrode, causing the internal piston to leak brake fluid. This leak will cause the brake pedal to feel soft.

Bad wheel cylinders

Some cars are equipped with disc brakes on the front wheels and drum brakes on the rear wheels. Drum brake systems feature a drum that rotates with the wheel. Inside of the drum is a set of brake shoes that is forced against the drum by hydraulic fluid created by the wheel cylinder and pressing the brake pedal, which will cause the wheel to slow down.

Worn master cylinder

The master cylinder holds the brake fluid and feeds it to the front and rear brakes. Unfortunately, the master cylinder can wear out. When it does, it can leak brake fluid, causing a drop in hydraulic pressure to the brakes.