Why won't my taurus fully brake?

I have a 96 ford taurus, and recently having problems with the brakes. I would push the brakes, and I could hear air coming out, so I changed the pads up front, and still. I changed the master cylinder and the booster, which were messed up, and it helped, but very little. Bled all 4 sides, and the cylinder too, and still. I wonder, could it be the rear pads? Or, whatever they are called, I know up front they are pads, the rear ones, not sure what they are called, but I didn't change those rear ones. So, I really need help here, because if it is just that, then ok, I have the rear ones and have someone who can install them. But if it is not that, what could it be?