The most likely cause is the master cylinder was damaged when 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. When the pedal is pushed over half way, whether pedal-bleeding with a helper, being surprised by a sudden leak, or when pumping the pistons out of the calipers to adjust them, the rubber lip seals are run over that crud and they can be ripped.
Also consider that if the vehicle has anti-lock brakes, and the master cylinder was allowed to run empty while the caliper was off, it may be necessary to use a scanner to bleed the chambers in the hydraulic controller. The scanner will let you command the computer to open some of the valves so the air can be expelled.
Sunday, May 7th, 2017 AT 6:04 PM