What happened to the troubleshooting session? All I have read is guesses. You loosened the master cylinder and pulled it away from the booster, and the problem didn't go away. The next thing I would do is crack the line at the master cylinder that goes to the front brakes. Most vehicles have the lines swapped, the rear one goes to the front brakes and the front line goes to the rear, but check yours to make sure.
If you crack the line at the master and the brakes release, there is something wrong with the master cylinder.
If cracking the line didn't release the brakes, go down to the caliper and crack the bleeder open. If it releases, then you might have a line clogged or kinked. If it never releases, then I agree you might have a parts compatibility problem.