* Air in the brake lines. This is the most common cause of spongy brakes. Air can get into the lines when the brake fluid is changed, or when a brake line is repaired.
* Low brake fluid. When the brake fluid is low, there is not enough fluid to pressurize the brake system and firm up the pedal.
* Worn brake pads. When the brake pads are worn, they can no longer provide enough friction to stop the vehicle. This can cause the pedal to feel spongy.
* Damaged brake rotors. If the brake rotors are warped or damaged, they can cause the brake pedal to vibrate or feel spongy.
* Weak brake booster. The brake booster is a vacuum-powered device that helps to amplify the force applied to the brake pedal. If the brake booster is weak, it can cause the pedal to feel spongy.
Solutions
* Bleed the brake lines to remove any air. This can be done by following the instructions in your vehicle's owner's manual.
* Add brake fluid to the reservoir. The brake fluid level should be between the "min" and "max" marks on the reservoir.
* Replace the brake pads and/or rotors if they are worn. The brake pads and rotors should be inspected every time the vehicle is serviced, and replaced as needed.
* Have the brake booster tested and repaired if necessary. The brake booster is a critical safety component, and should be repaired or replaced if it is not working properly.
Spongy brakes can be a serious safety hazard, and should be investigated and repaired as soon as possible.
