Have you checked the brake safety switch? If the o-rings in it are no good, the brakes won't work very well. To check it, just take out the switch. If there is fluid behind the switch, the o-rings are no good. If no fluid, it is fine.
This was the problem on my Dad's Cuda. We could not get the brakes to lock up (disc/drum). We tried everything, new master, booster, fluid, rebuild calipers and wheel cylinders. Nothing worked until we rebuilt the safety switch. Brakes work great now.
Here is a link on how to rebuild.
Link