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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.