Sounds to me that either the master cylinder is faulty or there is a breech in the system somewhere. I have only experienced bad pedal with 1. bad master cyl. 2 bad wheel cylinder 3. air in lines 4. broken line or loose fitting. Take the rear tires off and check the backsides of rim for brake fluid. For $15 another master Cylinder is a good preventive maintenance investment as well as insurance. If it is not the master, move on.

I read your post again..It sounds to me you have replaced all possible parts which would cause this condition except for the master and brake lines..try a new master..I have even purchased them new and rebuilt and have had them go out soon after replacing. If the car saw little use after replacing, that can sometimes cause a MC to go bad.