do you mean as you step on the pedal it continues to slowly go to the floor? If so that would indicate a leaky wheel cylinder(s) or master cylinder. Check rear wheels first for any signs of brake fluid leaking out of wheel cylinders if none replace master cylinder.
If the fluid in the master cylinder looks brown, you may have contaminated brake fluid. I had this happen on a tow vehicle many years ago. On a downgrade the water in the lines boiled and the pedal went to the floor. After letting it cool off, the pedal felt fine.
Service need: Brake fluid becomes oxidized and contaminated over time and use. Heat and moisture cause corrosion on parts and brake fluid failure. This causes a spongy brake pedal and shows up in discolored brake fluid.
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