My 2017 Panda 4x4 TA had its master cylinder replaced under warranty because the pedal went to the floor and only came up if lifted. It did sort of work for a few days before it was changed but felt odd. The return spring for the pedal is inside the cylinder and can fail. Sounds as if this might be your problem. Changing the master cylinder was done in about 1.25 hours if I remember right.
lighthouse19