A brake pedal that feels soft, spongy, or sinks toward the floor is never something to ignore - but some causes are urgent and others are merely annoying. Here is how to tell the difference and when to stop driving immediately.
Stop driving now: a sinking pedal
If the pedal slowly sinks to the floor under steady foot pressure while you sit at a light, that is a red-flag symptom. It usually means fluid is escaping - either an external leak at a caliper, hose, or line, or an internal leak past the master-cylinder seals. Either way your hydraulic system is failing and can lose braking entirely. Do not drive it; have it towed.
Urgent: pedal goes low then firms on a second pump
If the first pedal press is low but a quick second pump brings it up firm, you likely have air in the lines or badly worn pads. Air is the most common cause of a spongy pedal and calls for a proper bleed in the correct bleeding sequence. Check your pads too - see when to replace brake pads. Get it fixed within days, not weeks.
Common causes ranked
From most to least dangerous: a fluid leak (urgent), a failing master cylinder (urgent), air in the lines (soon), old moisture-laden fluid that boils under hard use (soon), and a swollen soft brake hose (soon). Old fluid is a frequent quiet culprit - see brake fluid guide for why moisture ruins pedal feel.
What to check first
Look at the fluid reservoir level - a low level points to a leak or worn pads. Check the ground under the car for wet spots. If the fluid is low or dark, or the pedal sinks at all, treat it as urgent. Fresh pads and hardware plus a full flush restore a proper firm pedal on most cars.