Soft or spongy brake pedal: why it is urgent

What it means

The brake system is hydraulic. The master cylinder converts the force from your foot into pressure in the brake fluid, and the brake lines carry that pressure to each wheel. For that to work, the fluid has to stay free of air bubbles and excess moisture.

Brake fluid absorbs moisture from the air over time, even in a sealed system, and that lowers its boiling point. If it boils under hard braking, it can form vapor bubbles in the lines. Unlike fluid, vapor compresses, so the pedal feels soft or goes further to the floor than expected.

Common causes

• Air in the brake fluid
• Moisture in the fluid, which lowers its boiling point and can form vapor under hard braking
• Another fluid or hydraulic issue in the system

What to do now

1. Do not keep driving and wait to see. Have the brakes inspected before driving further.
2. Check the brake fluid level in the reservoir against the min and max markings.
3. If the level is low, do not just top it up. A dropping level can mean worn pads or a leak, and the difference matters.
4. Look for visibly cracked, bulging or leaking flexible brake lines, where you can see them.
5. Note whether the brake warning light is on as well.
6. On a manual car, mention it if the clutch pedal also feels soft.

Soft pedal, or something else?

• Soft or spongy, goes further to the floor than expected: possible air or moisture in the fluid, so get it checked immediately
• Softer during repeated hard braking: moisture-contaminated fluid boiling into vapor
• Brake warning light on and the pedal feels different: worth having checked promptly
• A pulsing pedal during normal braking: usually uneven rotor thickness, so get it checked soon
• A pulsing pedal with grinding or clicking under hard braking: ABS working, which is normal
• Electric or hybrid car, slightly inconsistent pedal at low speed: usually the blend of regeneration and friction braking, not a fault

What people often get wrong

That brake fluid which looks fine is fine. Moisture contamination is not visible, which is why most manufacturers recommend flushing on a time-based interval, commonly every two to three years, regardless of mileage or how the fluid looks.

When the car must go to a workshop

Always. Diagnosing a soft or spongy pedal, and bleeding or flushing the brake fluid, are jobs for a professional. Air in the lines is a serious safety issue, and even experienced home mechanics leave bleeding to a shop with the right tools.

Read next

• Brake warning light: check this first
• Brakes squealing or grinding: what the noise means
• ABS light on: do the brakes still work?
• How to check brake fluid, and when it needs changing

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.