The vacuum will not cause a spongy feel to the pedal. That would only be caused by air in the hydraic brake line. I have seen master cylinders go bad during bleeding and the first thing you should have done was bleed the master.
All the power booster does is reduce brake pedal effort. The pantera needs about 14 inches to operate the booster as designed. It will give you boost at 12 inches but at a reduced amount.
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