A brake pedal that sinks to the floor or feels mushy is a hydraulic problem and a safety problem. Air, fluid, or a failing master cylinder is almost always the cause. Here are the most likely culprits ranked by how often they turn out to be the issue.
Air compresses; fluid does not. Even a small bubble after a brake job or fluid loss makes the pedal feel spongy and travel far.
A leaking caliper, wheel cylinder, or hose drops fluid level and lets air in. Check for wetness behind each wheel and under the master cylinder.
Internal seals leak so pressure bleeds back instead of pushing the calipers. Pedal sinks slowly to the floor with steady foot pressure.
Pads worn past the wear indicator or rotors deeply scored cause excess pedal travel as the caliper extends to reach metal.
A failed vacuum booster makes the pedal hard, not soft - but a leaking diaphragm or check valve can mimic soft pedal as vacuum bleeds away.
The pedal goes to the floor at all, you can barely slow the car, or the brake warning light is on. Do not drive. Tow the car or call for service - sudden total brake loss can happen.
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Air in the lines (most likely after a brake job) or a failing master cylinder leaking internally. A pressure test rules out external leaks.
A bleed is $80-$200. A master cylinder is $250-$700 installed. A leaking caliper runs $200-$500 per side.
No. The pedal will get worse, not better, and total brake failure is possible. Tow it to a shop or fix it before driving.
Yes. Brake fluid absorbs moisture, which lowers its boiling point and corrodes internal parts. Flush every 2-3 years or 30k miles.
The pedal sinks slowly to the floor when you hold steady foot pressure at a stoplight. The brakes still work but the pedal feels deep and unsafe.