Symptoms
- Pedal held at a stop light gradually sinks lower without moving your foot
- May need to occasionally re-press the pedal to maintain firm braking
- Possible slow external leak or none visible at all
Likely causes
- CommonInternal leak in the master cylinderWorn internal seals let fluid bypass internally instead of building pressure, causing a slow sink.
- PossibleExternal leak in a line, hose, or caliperFluid escaping externally reduces pressure over time even while holding the pedal.
- Less commonAir trapped in the system from a previous incomplete bleedCan sometimes cause a slow sink, though a leak is more common.
What to do
- Test this again at a full stop, holding steady pressure on the pedal and watching for gradual sinking.
- Check under the car and near the master cylinder (usually at the back of the engine bay) for any wetness.
- Check the brake fluid reservoir level for any drop.
- Avoid driving the car if the sinking is significant or fast, since this points to a real hydraulic problem.
- Have a mechanic pressure-test the system and inspect the master cylinder specifically.
- Have the master cylinder replaced if it's found to be leaking internally, then have the system bled.
Tools and parts
Master cylinderSearch: “master cylinder for your car make and model”
When to call a pro
See a mechanic promptly to check the master cylinder and hydraulic system, since this is a sign of developing brake failure.
Safety
Use caution
A slowly sinking pedal at a stop can turn into brake failure while driving; have this checked before continuing to rely on the car for regular trips.