1991 Ford Mustang V8 Two Wheel Drive Manual 126000 miles
I've replaced just about everything I can. Still, my brake pedel goes to the floor. I have replaced the brake booster, master cylinder, brake proportion valve, brake lines, rear master cylinders, front calibers and bled my brake lines. None of this has helped. The pedal is hard, but still goes completely to the floor.
You may have a faulty new master cylinder, this is not that uncommon, try this first, get 3 brake hose clamps and clamp off all brake hoses, check that the pedal is hard & high, if not the master is at fault or there is still air in the lines up stream of the clamps, if pedal is hard the problem is down stream of the clamps.