Causes of a Soft Brake Pedal
1. Air in the brake line(s)
Your vehicle's brakes rely on a hydraulic pressure system to operate, and brake fluid helps to maintain that pressure. Brake fluid is the only thing 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 spongy. A process called “bleeding the brakes” gets rid of the air. Air in the brake line(s) is the most common cause of a soft brake pedal.
2. Damaged brake line(s)
Since the brake lines are made of steel tubing, they can become corroded by rust. Over time, the rust can cause small holes to develop and cause the inside of the hose to collapse 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. A collapsed brake line will cause a restriction in the flow of brake fluid and a loss of hydraulic pressure at the calipers and/or wheel cylinders.If your brake pedal feels spongy, it's a good idea to check the brake lines for leaks. If you find a leak, have the brake line(s) replaced immediately.
3. Bad disc brake calipers
Like brake lines, disc brake calipers (the piece that clamps the brake pad down on the rotors to slow the wheels down) can also become corroded, causing the internal piston to leak brake fluid. It's that leak that will cause the brake pedal to feel soft. If you suspect that a caliper might be corroded, have your vehicle seen by a qualified professional as soon as you can.
4. Bad wheel cylinders
Some cars come equipped with disc brakes on the front wheels and drum brakes on the rear wheels. Drum brake systems feature a drum that rotates along with the wheel. Inside the drum is a set of brake shoes that is forced against the drum by hydraulic fluid created by the wheel cylinder and the brake pedal being pressed, causing the wheel to slow down. Sometimes corrosion inside that wheel cylinder can cause brake fluid to leak, resulting in a loss of hydraulic pressure, which in turn leads to a soft brake pedal. In this case, it is integral that the system be evaluated as soon as possible as this event is symptomatic of requiring brake part replacement.
5. Worn master cylinder
The master cylinder is the heart of the brake system – it is what 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. A soft brake pedal is the result of a loss of hydraulic pressure. You should have your vehicle seen by a mechanic immediately if your master cylinder is worn out or leaking.