Glad we are making progress...
Does the pedal "pump up" when you are pumping the pedal, or does it stay at the same height every time you push it?
If you hold firm pressure on the pedal does it sink to toward the floor?
How high above the floor is the pedal when you are pushing on it hard?
If it stays the same and doesn't sink down, then try this...
Go into a gravel parking lot and drive about 15 mph and hit the brakes hard. If any wheels lock up, you will be able to see by the ruts in the gravel which wheel or pair of wheels it was.
Let us know what you find.