A spongy pedal almost always means air in the hydraulic system. Brake fluid doesn't compress, but air does. So when you push the pedal, the air squishes and the pedal feels soft. Pump it and it firms up for a bit.
Here's how air gets in, why it sometimes won't come out, and how to tell spongy from other pedal problems.
How air gets in
The usual way is a repair that opened the system, like replacing a caliper. If the pedal went spongy right after, the fluid's full, the booster works, and there are no leaks, it's air. Bleed it the way service information says.
When bleeding doesn't fix it
If the pedal stays spongy no matter how many times you bleed it, something's keeping the air trapped.
- Calipers on the wrong sides: air rises to the top. If the calipers got swapped side to side, the bleeders end up on the bottom and can't get the air out
- ABS hydraulic unit: some internal passages only open when the valves and pump run. After replacing the unit, you may need the scan tool's automated bleed while bleeding the wheels
Spongy vs low vs sinking
These feel different and point to different problems:
- Spongy, firms up when pumped: air in the system
- Low but firm, getting lower over time: on drum brakes, think seized self-adjusters letting shoe clearance grow
- Slowly sinks under steady pressure, no leaks, fluid level doesn't drop: fluid is slipping past the seals inside the master cylinder
Testing a sinking pedal
To prove it's the master cylinder, plug its outlets so it's cut off from the lines and wheels. If the pedal still sinks with no fluid lost, it's bypassing internally. Replace it, bench bleed it, and bleed the system.
New drum shoes, low first pedal
If new shoes went in with the adjusters backed all the way off, the first pedal is low because the shoes have to travel out to the drums. Pump it and it comes up. Adjust the shoes to the drums before the car goes back out.
Cheat sheet
- Spongy pedal that firms up when pumped = air
- Bleeders have to be on top, so check the calipers are on the right sides
- New ABS unit may need a scan tool bleed
- Low but firm pedal on drums = check the self-adjusters
- Sinking pedal with no leaks = master cylinder bypassing
Common questions
What's the most common cause of a spongy brake pedal?
Air in the hydraulic system. Bleed the brakes the way service information says.
Why is my pedal still spongy after bleeding?
Something's trapping air. Check that the calipers aren't swapped side to side, and see if the ABS unit needs a scan tool bleed.
Why does my brake pedal slowly sink?
If there are no leaks and the fluid level doesn't drop, the master cylinder is likely bypassing internally.
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