If your brake pedal slowly sinks towards the floor when you are stopped with your foot on the brake, or the pedal does not feel firm when the brakes are applied and slowly sinks, your brake system may have a fluid leak or your master brake cylinder is worn out or leaking. Either way, your brakes need immediate attention.
If the brake warning light is NOT on and there are no visible brake fluid leaks, your master cylinder may be worn or leaking internally allowing the brake pedal to slowly sink when pressure is applied to it. This type of condition will be most noticeable when you are holding constant pressure against the brake pedal at a stop light. If the pedal sinks or requires pumping to keep your car from creeping ahead, the master cylinder needs to be replaced.
On some vehicles with rear-wheel antilock brake systems (ABS), it's also possible that a leak in the ABS unit may cause a similar sinking pedal condition.
If the BRAKE warning light is on, you most likely have a brake fluid leak. Your vehicle may not be safe to drive in this condition! You should have the brakes inspected as soon as possible to determine where the fluid is leaking (usually a hose, brake line, brake caliper or wheel cylinder) so the necessary repairs can be made. Wet spots around hose or line connections, or fluid leaking from a disc brake caliper or drum wheel cylinder would indicate a serious problem that needs immediate attention.