Master Cylinder: Pedal Fade
Leaking master cylinder cup seals result in pedal fade. The master cylinder consists of two pistons that work in tandem to apply pressure to the braking system. These pistons use cup seals to seal fluid in the pressure chamber. When the brakes are not applied, the seals rest between the vent port and replenishing port. As the piston moves the seal past the vent port, pressure is created in the pressure chamber through the lines and onto the braking units.
If the fluid pressure bypasses the cupped seal on either the primary or secondary piston, the pedal will fade toward the floorboard. It will have to be continually pumped to maintain pressure in the circuit. These seals can wear or become contaminated. This problem is so common it's important to make sure and rule out any external leaks before repairing or replacing the master cylinder.