Cause
Leak in the brake hydraulic system
Fluid escaping from the brake hydraulic circuit. It is the fault on this list that most directly affects whether the vehicle stops, and it is why a falling fluid level is never something to top up and move on from.
Modern systems are split into two circuits so that one leak leaves some braking, which is what makes a leak survivable — and also what lets it go unnoticed, since the pedal travels further and the vehicle still stops. A pedal that sinks slowly under steady pressure is an internal leak, usually the master cylinder, with nothing visible outside. Look at the flexible hoses, the caliper seals and the pipe unions, and inspect the rear of the master cylinder where fluid can leak into the servo rather than onto the ground.
Typically specialist work. Needs equipment or training most home workshops do not have.
Some of these codes mean you should stop driving
1 of the 1 codes this can cause indicate a fault where continuing to drive risks damage or is unsafe. Which one you have matters more than the cause does — read the code's own page before deciding.
Codes this can cause (1)
Codes where this is a likely explanation first, rather than merely a possible one.
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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