* Loss of brake fluid: This is the most common and dangerous cause. A leak in the brake lines, master cylinder, wheel cylinders, or calipers will allow brake fluid to escape, resulting in a spongy or completely collapsed pedal. Look for leaks under the car, especially around the wheels and near the master cylinder (usually under the hood).
* Master cylinder failure: The master cylinder is the pump that pressurizes the brake fluid. Internal failure within the master cylinder can prevent it from building pressure.
* Air in the brake lines: Air in the brake lines is compressible, meaning the pedal will go to the floor as the air gets compressed instead of building pressure to stop the car. This usually happens after brake work, but can also occur due to a leak.
* Brake booster failure: The brake booster uses engine vacuum to assist in braking. If the booster fails, you'll have very little braking power, resulting in a soft pedal.
* Wheel cylinder or caliper failure: A leak or internal failure in one of the wheel cylinders (on drum brakes) or calipers (on disc brakes) can cause a loss of braking power at that wheel, but it could also lead to a soft pedal across the board.
* Severely worn brake pads or shoes: While usually causing a firmer but less effective pedal, extreme wear could contribute to a soft pedal if it's combined with other issues.
What to do:
1. Do not drive the vehicle. Attempting to drive with a brake pedal that goes to the floor is extremely dangerous.
2. Have the vehicle towed to a qualified mechanic. This is not a problem you can safely or effectively diagnose and fix yourself.
3. Describe the problem clearly to the mechanic. Tell them the pedal goes to the floor when the engine is running.
Ignoring this problem could lead to a serious accident. Safety should be your top priority.
