Originally Posted by Skippy

The original problem was most likely caused by either bad calipers or bad hoses. Since you've replaced those, you shouldn't be having that problem anymore. Did you use all new parts aside from the master cylinder?

Diagnosing a bad master cylinder is a process of elimination. If your pedal is going to the floor and you aren't losing fluid, you either still have air in the system or the internal piston seals in the master cylinder are bad. I would bleed the brakes again. It's possible Firestone may have done it wrong. If you still get the pedal going to the floor, the next step would be to replace the master cylinder.