A brake pedal that goes to the floor means the system is losing hydraulic pressure. The most common causes are a fluid leak (at a brake line, hose, caliper, wheel cylinder or master cylinder), a master cylinder that leaks internally, and air in the lines. A pedal that feels soft or spongy but stops the car usually means air in the system or old fluid. This is a safety problem: do not drive until it is fixed.
Diagnose by symptom
| Symptom | Likely cause |
|---|---|
| Pedal sinks to the floor, fluid level dropping | External leak: lines, hoses, calipers, wheel cylinders |
| Pedal slowly sinks under steady pressure, fluid full, no leaks | Internal master cylinder leak |
| Spongy pedal that firms up when pumped | Air in the lines; see how to bleed brakes |
| Pedal low after a pad change | Caliper pistons pushed back; pump the pedal before driving |
| Hard pedal that needs heavy force | Brake booster or vacuum supply, not a leak |
Goes to the floor but the fluid is full
With no leaks and a full reservoir, a pedal that sinks while you hold steady pressure points to the master cylinder’s internal seals letting fluid bypass. A pedal that sinks while idling at a light is a classic sign.
Goes to the floor then pumps up
A pedal that gets higher with each pump usually has air in the system, or a lot of clearance from pistons that were pushed back. After bleeding, a spongy pedal means air is still trapped, the bleeding order was wrong, or the ABS unit needs a scan-tool bleed on some vehicles.
Brake fluid leaks
Brake fluid is clear to amber when new and darkens with age; it feels slick and is often found inside the wheel, at a caliper, along a rusted steel line, or under the master cylinder at the firewall. A wet tire sidewall or a damp backing plate on drum brakes points to that wheel. Rusted lines are a common failure on older trucks in salted regions.
If it happens while driving
- Pump the pedal quickly to build what pressure you can.
- Shift to a lower gear to use engine braking.
- Apply the parking brake gradually so the rear wheels do not lock.
- Steer to a safe place and stop.
Old fluid
Brake fluid absorbs water over time, which lowers its boiling point and can cause a soft pedal after heavy braking. Replace it on the schedule in your manual. Related: ABS light, sticking brake caliper and how long brake pads last.
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