Brake problems

Two common things to look for. If the vehicle has anti-lock brakes, and the master cylinder's reservoir ran empty, you will likely need a scanner to access the ABS Computer and command it to open the valves so some chambers can be bled. Many newer scanners have the procedure built into the software, and you just follow the steps displayed.

The second thing for any vehicle more than about a year old is the master cylinder was likely damaged the first time the brake pedal was pushed all the way to the floor. Crud and corrosion build up in the lower halves of the bores where the pistons do not normally travel. Pushing the pedal over half way to the floor runs the rubber lip seals over that crud and can rip them. That results in a slowly-sinking brake pedal or one that does not build pressure, and that often takes two or three days to show up.