Sitting at a red light, you notice the brake pedal slowly creeping toward the floor even though you are holding steady pressure. That specific symptom is an important warning. Here is what it means.
The classic master cylinder sign
A pedal that sinks under constant pressure - the hallmark of stop-and-go traffic - usually means the master cylinder is bypassing internally, letting fluid slip past worn seals instead of holding pressure; see why the pedal sinks slowly. It often does this with no visible leak.
Or an external leak
A leak at a caliper, hose, or line also lets the pedal drop as fluid escapes; see brake fluid leak signs. Check the reservoir - a dropping level plus a sinking pedal points to a leak rather than the master cylinder.
Why traffic reveals it
The problem shows up in traffic because that is when you hold steady pedal pressure the longest. A quick stop may feel fine, but holding at a light lets the slow leak-down become obvious.
Treat it as urgent
A pedal that will not hold is a failing hydraulic system that can worsen to lost braking - do not put it off; see brake service. Have the master cylinder and lines inspected. Keep the whole system healthy with quality pads and hardware.