Brake pedal soft or spongy
A spongy pedal means something compressible is in the hydraulic line, almost always air or vapour from old fluid, or a mechanical component is flexing before the pads bite. A pedal that is soft but firms up when pumped is air; a pedal that sinks slowly under steady pressure with no external leak is a master cylinder bypassing internally. Treat any sudden change as a stop-driving fault until the cause is found.
Urgency
Stop driving
Possible causes, most likely first
- Air in the hydraulic system
Pedal travel is long but firms up after two or three pumps. Usually follows a caliper or hose change, or a reservoir that ran dry. Bleed all four corners in the correct sequence, ABS unit included where the procedure requires a scan tool. - Old, water-saturated brake fluid
Fluid absorbs moisture and boils under hard braking, giving a pedal that goes soft on descents and recovers when cool. Test with a boiling-point or moisture tester; above 3% water or below 180 degrees boiling point means change it. - Master cylinder internal leak
With the engine running, hold firm pressure on the pedal for 30 seconds: a pedal that creeps slowly to the floor with no external leak means the master cylinder seals are bypassing. - External fluid leak
Reservoir level dropping and a wet patch at a caliper, wheel cylinder, hose or pipe union. Inspect each wheel inside the rim and the pipes along the sills; fix any leak before the car moves again. - Swollen or perished flexible brake hose
Old rubber hoses balloon under pressure, giving a spongy feel without air in the system. Have someone press the pedal hard while you watch and squeeze each hose; any visible swelling or cracking means replace all of them. - Rear drum brakes out of adjustment
Excess shoe-to-drum clearance uses up pedal travel before the front brakes work properly. Adjust the self-adjusters or shoes and check the handbrake travel, which is usually too long as well. - Faulty brake servo or vacuum supply
A pedal that is hard rather than spongy, or that sinks when the engine is started, points to the servo. Pump the pedal with the engine off, hold it and start the engine: it should drop slightly; no drop means no vacuum assist.
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What you can check yourself
- Check the brake fluid level and colour; dark brown fluid is overdue for a change
- Pump the pedal three times at a standstill: if it firms up, there is air in the system
- Hold steady pressure on the pedal for 30 seconds with the engine running and watch for it sinking
- Look inside each wheel rim for fluid streaks and check the ground where the car is parked