Sitting at a red light with your foot on the brake, you notice the pedal slowly sinking toward the floor, or the car trying to creep forward. That specific symptom is a recognized warning sign. Here is what it usually means.
A sinking pedal at a stop
If the brake pedal slowly sinks under steady, constant foot pressure while you sit at a light, the classic cause is a failing master cylinder - its internal seals are bypassing, letting pressure leak past so the pedal creeps down; see when a soft pedal is urgent. This is a genuine safety concern and should be diagnosed promptly.
Or a fluid leak
A slow external fluid leak can also cause a pedal that gradually sinks and a reservoir that keeps dropping - see brake fluid leak signs. Either way, a pedal that will not hold firm under steady pressure means the hydraulic system is not sealing.
Creeping forward: idle or transmission
If it is the car physically creeping (not the pedal sinking) while the brakes hold, that can be normal automatic-transmission creep at idle that just needs firmer pedal, or a high idle. If you must press harder than usual to hold the car, though, check the brakes and the brake booster assist.
What to do
A pedal that sinks under steady pressure at a stop is a stop-and-diagnose symptom - suspect the master cylinder or a leak and get it checked before driving far. Keep the whole system healthy with fresh fluid, quality pads, and hardware.