If your vehicle is showing brake pedal sinks slowly to the floor while stopped at a light, here are the most likely causes and the parts you'll need to fix each one.
The internal rubber seals inside the brake master cylinder are worn out. When you hold steady pressure on the pedal, brake fluid slowly bypasses the seals internally, causing the pedal to sink without any visible fluid loss.
If brake work was recently performed, air may not have been fully bled from the system. Air compresses much easier than brake fluid, causing a spongy pedal that can slowly sink under pressure.
A pinhole leak in a rusted brake line, cracked rubber hose, or failing caliper seal is allowing fluid to escape under pressure. This will typically be accompanied by a dropping fluid level in the reservoir.
The internal dump valves inside the Anti-lock Braking System (ABS) module can fail to seat properly. This allows hydraulic pressure to bleed off internally back into the ABS accumulator, causing the pedal to drop.